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


  
  Fill a container with water deep enough to easily submerge your food and bag.
  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.
  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.
  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.
  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.
  Just as the open part of the seal is about to go under the water, close it up.