Do I need to "unroot" my Evo 3D to get the ICS update?
ROM Updates from vendors/manufacturers/carriers usually evaluate checksums of at least the system partition -- which fails in case of applied modifications, such as required by rooting (rooting needs to place at least the superuser apk on the system partition, and other root-apps may then add to it). I'm not sure how this is handĺed by HTC for devices "unlocked" via their "service" -- but in all cases I experienced, OTA-Updates failed due to the CRC check.

"Un-rooting" will probably not help much: the system partition remains "tainted". The only thing I know which should definitely help is flashing the original ROM locally.

As for me, I ended up using CyanogenMod on my devices, which runs quite smoothly (HTC Buzz as well as Motorola Droid 2). If you either have to flash (the original stock ROM), you might want to give CM a try. A good backup with Titanium Backup should take care for your data and make migration easier.