With the DHD a stock Android 2.2 follows, with various HTC and HTC sense apps.
There is a scheduled update to Android 2.3/2.4 (Gingerbread) at the end of Q2, but when exactly there is no word on yet.
As for updates, there was a OTA update to 2.2 to 2.2.1, but that was pulled very fast (basically a few hours after release) as they found many serious bugs. It did have great upgrades for the OS itself, but many problems surfaced, like the "sms disappearing" bug. Thus there is no scheduled update for the DHD before 2.3/2.4 comes out from HTC
Hope that helped