If you just can't wait to get your hands on the latest and greatest Google has to offer, then XDA Forums member preludedrew has just the thing you're looking for. He has crafted the Google Gingerbread source code into a bootable, working custom ROM for the HTC EVO 4G. Now before you jump up and down and shout hallelujah, be aware that this is a very early release, and probably not quite ready for full everyday usage.
However, if you are interested in giving it a run, we've got a direct download and link to the XDA Forums thread after the break.
I've been running this ROM for a couple of hours, and so far haven't really experienced any major problems with it. However, your mileage may vary. This is very much a beta release of this ROM (more like an alpha release), so there are no guarantees that you won't run in to issues with it.
Here's the link to download the current version as of the writing of this article.
And here's the link to the developers thread over at XDA. I strongly recommend checking that thread to see if there are any updates for the ROM.XDA Developers via GoodAndEvo.net