You didn't really think the best Android smartphone to date would be released just two weeks after the official announcement of Android 2.2 (also known by its codename, Froyo) and then not be eligible for the update, did you? Google definitely wouldn't have handed out free HTC EVO 4Gs to its I/O conference attendees if it was going to be stuck on Android 2.1 forever.
But since it's never wise to rely on assumptions, Android Central has confirmed with HTC that the company "will most likely offer an upgrade" for the EVO 4G (and other Android phones released in 2010) "in the second half of this year." Now granted, "most likely" isn't exactly a solid promise, but it's better than a flat-out no.
HTC will announce a list of eligible phones and their respective upgrade dates after everything is sorted out with Google and other partners. The company's custom Sense overlay will probably be at the center of most discussions (and maybe even the cause of any delays); HTC is currently working hard to get "the earliest access to everything [needed] to provide a complete and solid Sense experience on Froyo."