Is it possible to do sous vide cooking in a crock pot?
Yes you can.  Some of the best sous-vide I've had has been in a crock-pot.

You'll need a crock-pot with a manual (analogue) dial. What you do is set the dial to maximum and use a PID controller (found ~$20 on ebay).

Take a look at this article about hacking your slow cooker:

http://www.cookingforgeeks.com/blog/posts/diy-sous-vide/

Update:

Sous-vide by definition requires control of the temperature (Beer Cooler does not control, but holds well).  However, you can still do this without extra logic.  All you need is a good thermometer and time:

Fill your crock-pot to 70% with room temperature water, set your Crock-Pot to the lowest setting, and leave it for 2-3 hours.  Thermo-dynamics laws dictate that you will reach an equilibrium with time.  If you're bored you can keep a log of the temperature change over time and stop when you notice it doesn't change much any more (it'll be helpful for next cooking session).

Measure the water temperature and check against where you need to be (125, 140, etc).  Adjust the dial and throw more time at it until the crock-pot reaches equilibrium again.  Repeat until you have reached and sustained your desired temperature.  Make sure your crock-pot is not near an open window with draft to mess with the crock-pot (i.e. keep the room temperature steady).

Weigh your food and put it in the rock-pot.  Take out the same amount of water from the existing water in the crock-pot so that the total weight of the crock-pot is the same.  

This isn't hard if you use the metric system.  Each Millilitre of water weighs 1 gram.  So for 1lb, take out 454ml or essentially two cups.

Now you can use the same timing and temperature rules as everyone else.

PS: For the 'ballistic' method, I.E. the beer cooler, you can measure the temperature loss over time of water in the cooler to land you perfectly where you want to be. Chef Kenji says a 'couple of degrees higher'.  But you can predict the drop over your cooking time as well as the initial temperature.  And roughly speaking: Mass Of Food x Food Temperature + Mass of Water * Water Temperature = Total Mass * Final Water bath temperature.