The Evo has an 802.11n-capable chip inside, but it's gimped to run slower 802.11g. If you've rooted your Evo (a serious task in itself), modders can help you remedy this limitation by changing just two lines of code.
Using a hex editor, you follow the instructions seen here—pretty much just copying and pasting some new code at just the right spot.
Of course, given how simple this fix is, it brings into question even more why the Evo didn't have its 802.11n enabled from the get-go. Most have speculated that battery life is to blame. That's possible, I guess, but I can't imagine it draws loads more power than 3G and 4G connections, especially running simultaneously. [XDA Developers via Object Oriented via Engadget]