You get a little blue icon that looks like a square lego question marks on the side. Most sites will tell you to download flash. Good luck getting that to work though.
There used to be a little bookmarklet that would enable flash videos to play in the video player. I'm not sure if it still works. It's called iTransmogrify.
http://joemaller.com/2008/01/12/itransmogrify/
I think Apple broke the bookmarklets. Very valuable bookmarklets like "find in page" and "enlarge text" were really useful and don't seem to work anymore. Stinking Apple. Bookmarklets never hurt no one and they had to cripple them just to be jerks.
Apple is lying when it says the reason against flash is battery life. The reason is clearly the same reason they won't give us MMS, C&P, and A2DP. It's completely illogical.