There's been a lot of complaints about the new Google Nexus One having problems staying on 3G with T-Mobile in the U.S. Early adopters are frustrated that they cannot get the issue fixed and don't have direct access to Google for customer service. I found that odd as Google provided phone support for their new DNS service, so go figure. Anyway, users are getting referred from T-Mobile to HTC and back again.
I doubt the problem is at T-Mobile. I've got the G1 on T-Mobile and it is an Android phone from HTC. The Nexus One is an Android Phone from HTC that works on T-Mobile's frequencies. So if a new phone with the same OS (Adnoird) from the same manufacturer (HTC) doesn't work on the same network (T-Mobile)... I wouldn't think it is the network, you know?
Both the G1 and N1 were branded as 'the Google Phone.' I figure that HTC builds both, but this time Google marketed it, so ultimately Google will have to lead the customer service charge. However, the problem is probably either a hardware issue (HTC) or a software issue (Google) that will need to be addressed. Hopefully, it is software, and an over-the-air update will fix it.
I don't have the N1 yet. :-( As a current subscriber, I don't get the subsidized price, so I didn't go for it (but boy is it tempting). But if I had the N1 right now, yes I'd be frustrated, but I also figure that Google will make it worth my while somehow.
Be patient, keep (politely) complaining -- they'll fix it soon.
Small Update: Steve Kondik (cyanogen) seems to think his N1 is bouncing between two 3G protocols, UMTS and HSDPA.