Search Engines and Javascript

It has been my understanding that search engines generally don't detect Javascript. I've been informed today this was incorrect. I wanted to confirm to see what is true and what is not.

In this particular instance I have a homepage that displays text (within div tags) for users that don't have Javascript and if Javascript is enabled it will replace the plain text with a Flash movie using a Javascript function.

From what I have been told Google can detect that I am doing this and it does not scan the text.

If this is true I find it strange the text is still showing up in the google results and when I made that change there was a significant change in ranking.

This brings up a lot of questions in my head. Any insight would be appreciated.

hmm thats partly true every real text within elements div, h, p, li, etc. will be indexed but be careful, check this advice from google

http://googlewebmastercentral.blogsp...-of-flash.html

i wouldn't try to use javascript for seo though... it might work but not as well as plain text...