The device will launch with Android 1.6 installed, and it appears that Android 2.1 will be completely bypassed, with Dell instead focusing on getting Android 2.2 out to the masses.
As for the reasoing behind the decision to not support T-Mobile, they don't really go into details, but they do say that the device shown is not certified as compatible with T-Mobile's 3G network in the US.
Android Central says "that's not necessarily to say it won't work on EDGE, it's just that nobody will be responsible if you fry a few brain cells in the process."
Here is a video showing Android 2.1 working on the Dell Streak:
Android Central also writes that "the presale window is still coming "this week" so get ready for launch day if you fancy this device and can't wait to get your hands on it. However, the device will only launch with Carbon Black, with New Cherry Red available later.
Phandroid adds that with this latest news, the device seems to be even more desirable:
If I were an AT&T fan I’d definitely be looking at picking one of these up if the Samsung Captivate wasn’t the superphone you were looking for. After being rooted and there being proof that a 2.1 version is out and about, you should have no reason to not pull the trigger if you were on the fence already.
Image from Android Central