The first tell-tale is a modified padlock picture in a system authorisation dialogue. If you see an odd-looking padlock, don't authorise. Prior to Lion OSX installed Java by default, and these users should take note.
If you are running Lion (and have not installed Java, or have always kept it switched "off" in Safari security prefs) there is little to worry about. Others may be interested in this article -- which looks intimidating but is simple enough when used to confirm that you DON'T have a problem. If you DO have a problem proceed with extra caution.
> http://www.f-secure.com/v-descs/trojan-downloader_osx_flashback_i.shtml <
Make sure you have read the whole article thoroughly, including "Additional Details" and then follow the instructions carefully, copying and pasting each Terminal command, checking the results and then moving to the next instruction indicated.
Hopefully Apple will by now have used an automatic virus/trojan fix to your Mac for this malware.
(Note that if you want to look inside the Safari.app (and other app bundles) you need to control-click the Safari icon in Applications, and look for the menu item "See Package Contents").