From Metro Studios Knowledgebase
 Functions.php Resources
- An Introduction to Functions.php in Wordpress
- 32 Extremely Useful Tricks for the Wordpress Functions File
 Common Problems
$ is not a function (see #5 http://www.ericmmartin.com/5-tips-for-using-jquery-with-wordpress/)
"All of the previous steps are nullified if proper jQuery coding conventions aren't followed. The most common issue I see with jQuery and WordPress is the misuse of the $ variable.
As a general rule, you should not use the $ variable for jQuery unless you have used one of the shortcuts. The following is an example of how to shortcut jQuery to safely use the $ variable: