AuxLV, a member of the xda-developers forum, modified the official Flash .apk from the Froyo-flavored Nexus One to work on Android 2.0 and 2.1, and it is said to be working on the EVO! Installing the modified app, which you can download here, replaces the EVO's preinstalled Flash Lite, so you don't have to fiddle with any settings to get it running. Just install the app from your microSD card (use ASTRO File Manager or appInstaller), restart your browser, and you should be good to go.
Results seem to be a little hit-or-miss right now, with some users running into problems and others enjoying all kinds of Flash goodies that they say were previously inaccessible from the EVO (e.g., Adult Swim, Facebook Flash games, various video streaming sites, Justin.tv, and so on). However, one member of xda says that the APIs needed for Flash 10.1 to work simply aren't present in anything but Froyo and that all of these "results" are really just from good ole Flash Lite.
Anyone try it yet?[xda-developers]