The HTC EVO 4G was rooted the same day that it was originally released -- and custom ROMs free flowed to the rooted devices shortly thereafter. Unfortunately, after you flash a factory ROM you typically loose your root access. It should come as no surprise, now that the Android 2.2 (Froyo) update is rolling out over-the-air (OTA) it, too, has been rooted.
First off, the following applies only to those who have already rooted their EVO. If you haven't, the following isn't for you. However, if you aren't rooted, but are interested in doing so, you can get all the steps you need at XDA-Developers -- don't worry, it's a very easy, "one-click" process to root your EVO.
Next, I don't have an EVO in-hand to get the precise steps for you (as I've done for G1 and Nexus One step-by-step guides), but the basics are the same for all Android's, only the buttons to be pushed are different.
For those of you who've already rooted, there's a little leg work involved, you will need to change your radios (regular and WiMax) before you apply the rooted Froyo ROM. Some users have reported success without wiping. As always, if you have any problems at all, try wiping.
Applying this ROM is just like any others, boot into recovery, flash the radios (rebooting when prompted), then flash the ROM. Both RA-Recovery and Clockwork have had reported successes.
Now, go and enjoy some Froyo!