Is a vacuum strictly necessary for sous-vide cooking?

Yep, Ziploc bags are fine although make sure you have ones that are appropriately heat sensitive for whatever temperatures you'll be cooking at. There's a good guide on the Cooking Issues blog:

http://www.cookingissues.com/primers/sous-vide/part-ii-low-temperature-cooking-without-a-vacuum/#sectionII3b1

1. Fill a container with water deep enough to easily submerge your food and bag.

2. Always add some sort of liquid to the bag –fat, stock, sauce, etc. The liquid is necessary to fill the gaps around your food and expel the air from the bag.

3. Add your food item. A significant advantage of Ziplocs over vacuum bags is that the food can be added to the bag hot. All vacuum bagging procedures require your food to be cold (more on that in the upcoming vacuum section of the primer). If you are searing meat and adding it directly to the Ziploc bag make sure the surface of the meat is below 100?C (212?F) or the bag will melt. Be especially careful to not touch the bag with a hot set of tongs or spatula.

4. Close the seal of the bag almost to the edge, leaving the last portion of the seal open –make sure you have correctly sealed the bag. Put your finger in the corner to make sure that part of the seal is open.

5. Carefully immerse the bag in the water starting with the closed corner, not the open one. Make sure you do this step carefully, allowing air to escape up and out of the open corner.

6. Just as the open part of the seal is about to go under the water, close it up.

For cooking a portion of meat sous-vide; use a zip lock style bag slightly bigger than the meat. Add the meat and zip the bag closed, but leave a very small opening. Then scrunch and roll the package with your hands to remove all the air, and then close fully the zip. Be careful not to tear the bag

Trying to do this in a bath of hot water is just silly. Your hands can exert much more pressure than a few cm of water could ever do

If the meat has bones sticking out; either use a sous-vide grade bag, or put a small strip of plain brown cardboard over the bone protrusion. After water bath cooking, make sure to discard the cardboard before finishing

As Joe mentions, if you have a cooking liquid to add to the bag, this task is even easier. Just fill the bag with meat and liquid, then let the bag rest on the bench until the liquid just starts to overflow the zip, and then fully close bag

For light foods, or foods where you can't get all the air out without the use of a vacuum pump; use a slightly larger bag than normal, and place some heavy objects in the bag (stainless steel food weights, or teaspoons) with the food. When this is placed in the water bath, the food will remain under water and in close contact of hot water/bag boundary, and the contained air will form a bubble at the top of the bag

I have used zip lock style bags, and haven't been too fussy with air extraction, and have had excellent results with sous-vide cooking

Creating a vacuum-like environment, by gradually submerging the bag in a water bath would seem to be the most efficient and effective way to evacuate the most air from the bag (leaving less pockets around the food). This method makes use of displacement effects, according to the Archimedes' principle. To me it seems intuitive, in the same way that a vacuum sealer is more effective than a straw. However it's not a point I would argue. That's just how it makes sense to me, seems to work out best, so I do it that way.

Sousvide Supreme explains that vacuum is used to guarantee even heat transfer:

The technique of sous vide cooking did not get the name ‘under vacuum’ accidentally. It relies on the efficient transfer of the heat from water to a water-based substance–ie, the food. Heat transfer through water is about 11 times more efficient than transfer of heat through air. Pockets of air between the water and the food impedes the transfer and can result in uneven cooking. Removing the air from the cooking pouch by vacuum sealing ensures that the food will be efficiently—and evenly—cooked in the time specified.?

So based on this, Ziploc is also good if you just get enough air out so that there are no pockets which would prevent the heat from transferring.

I've found that in the case of foods with a lot of liquid or sauce - slow cooked veal cheeks in my case - vacuum sealing just isn't an option without a professional vacuum rig. Ziploc bags should work but may leak. The easiest way I found and which works a treat, is simply to use a vacuum bag and leave the top open but folded and clipped to the side of the container so it wouldn't go under. Because there's liquid in the bag there's no problem with air pockets and because the cooking time is long anyway temperature distribution is just fine. If you're worried about it floating just clip a weight to the bottom of the bag.

If I want to SV something with liquid (sauce, stock etc), I use the water displacement method with a Ziploc bag, then put the Ziploc bag into a SV bag and vac seal it to ensure no leakages, especially if it is a long cooking period. I feel much happier that way.

The vacuum is required only the ensure the bag does not float in the water tank. If there is air inside the bag, the hot water will heat the air causing it to inflate and the entire bag to float thus resulting in uneven cooking. Therefore, it is better to vacuum the bag as much as possible so it remains submerged during the cooking period. Other technique is to put some weight on the bag to ensure it is kept under water yet this implies that the bag has to be completely sealed otherwise hot water will enter the bag and you will be boiling rather than "sous-vidding" so-to-speak. Hope this helps ....

The vacuum in the bag can only be reduced pressure, relative to atmosphere, as in a high vacuum, like outer space, all the liquids in the bag would 'boil' and the vapour produced have to be removed to maintain the high vacuum until only a dried husk remained. Therefore water bath pressure is a satisfactory method of expelling most of the free air from the bag.