The probe is tiny. The thing you have to be cautious of is if the wire thing the probe is connected to is so thin that a sharp lid or edge could cut it. So yes, it'll work fine with the lid on a typical slow cooker.

If you're using a slow cooker you might want to pour some hot water into it so you don't have to wait so long for it to heat up.

Oh! I just read about someone using Ball jars for sous vide. They must have a weight on them to hold them down. Or perhaps there's a way to pump out the air?

Ah neat! Foodsaver makes an attachment to seal them.

http://www.youtube.com/watch?v=lSKg...feature=related