Jason Ashdown's Blog of Wonders

I am currently in the middle of coding my first plugin for WordPress when I came across my first stumbling block…
Why doesn’t my jQuery code work in the admin area of WordPress??

After some probing on Google, the answer was revealed to me. Basically WordPress doesn’t use “$()” for jQuery, that’s reserved for ProtoType, which is also bundled with WordPress. Instead you have to use “jQuery(‘field’).” in order to get it working.

This article explains it in more detail.