Flash is a graphics animation program that uses vector graphics. Spiders have a limited ability to read Flash. Technology that enables multimedia to be embedded as part of a web page. Often used for introductions to web sites, games and animations. At this time, search engines do not index content that is within flash. As a result, from an SEO point of view, the use of Flash should be limited if you want to rank well in the organic search results of search engines such as Google and Yahoo.

Update:  As of June 2008, major search engines, in conjunction with Adobe, announced that they will now be able to index content that is found within Flash files. 

According to Google, "... Now that we've launched our Flash indexing algorithm, web designers can expect improved visibility of their published Flash content, and you can expect to see better search results and snippets."

More on Google Crawling Flash
More on Yahoo Crawling Flash