Ice Cream Sandwich was recently confirmed to be officially available in the future for the HTC EVO 3D and Design 4G, but not for the EVO 4G.
This left the flagship EVO by the wayside in planned obsolescence, but a custom ROM called Evervolv ICS for the EVO 4G sort of shot that plan in the foot.
A few days before ICS was freely released to the Android Open Source Project GIT, some developers were already working on a port using what they scraped off of other phones and on 11/11/11 there was a booting build of ICS for the EVO 4G. I stress "booting" because it had issues. It had more than issues, really; it had complete collections. It was a developer build and with all the problems it had, it was not ready for prime time. For the person looking for a daily driver, it's still not ready, but it's working.
Evervolv is probably the first functioning ICS-based ROM available for the EVO 4G, and while it's still really slim on features and fat on bugs, it works. It's a bit bloated and slow, but who isn't after Thanksgiving?
Currently the camera doesn't seem to work, some apps are prone to report IP6 addresses as opposed to IP4, there's no support evident for 4G, the WiFi is somewhat buggy in that it goes to sleep and never wakes up, and there are random force closes, but it's still a pretty fun play-with ROM.
The mail client seems to have received a makeover, and Exchange users will be happy to know they can subscribe to folders and have them updated automatically. The stock web browser has folders it stores web sites in instead of tabs. There are tons of nifty little tweaks that will breathe new life into your rooted EVO 4G once they get the bugs worked out.
You'll need to additionally flash the Google Apps for ICS if you want to do anything Market-related with this ROM, as well as install the Superuser binary from recovery. It's a lot of work to play with a stock ICS ROM that isn't complete, so keep that in mind. Supposedly the camera has been fixed with a new kernel build, but I have not had a chance to play with that at this time.
The developers of this 4G port have done an outstanding job so far getting this thing up and running, but remember that if you choose to install it that this is a developer alpha test. It's buggy, they know there are bugs, it's not ready for prime time, and not a daily driver. But it's an amazing little preview of ROMs to come for all of us.