Warning: This is so much fun you could waste hours or weeks playing with it!
Check out MadPattern, a set of free downloadable templates for Illustrator CS4/CS5 for "rapidly prototyping patterns". Watch the short video demo, download the templates, open one of the templates and follow the instructions on the pasteboard. Very easy and addicting!
See the MadPattern Gallery for some great examples of patterns created with the templates.
This Wikipedia entry explains the type of patterns that can be created with the MadPattern templates.
The emphasis in MadPattern is on exporting the finished patterns as bitmaps for Web use. But of course, the patterns being generated in Illustrator are vector, and with some ingenuity they could be used as elements in Illustrator vector projects.