I wonder if they have added more roaming agreements to their network. Last time I was in Norway, there was no coverage at all -- no network, no calls, never mind Internet access -- which I found strange. Usually you will be able to connect to via local network and pay for roaming charges. Norway is all GSM, same frequency band, and my plan has International calling, but I still had to swap out the SIM card for my Norwegian one. Could be a fluke, of course. Edit: Or it may be that pay-as-you-go doesn't provide international roaming.
I love T-Mobile's approach, and the whole reason I use them is because they are the underdog who's doing things a little differently.
I was in Norway as well and I did not get any reception on my T-Mobile phone until I contacted the account holder on our family plan (they were still in the US) and had them add international roaming to our post-paid plan. Immediately afterwards my phone was able to connect to a network, so I think roaming with T-Mobile is opt-in.
It does provide international roaming. At least in the Netherlands I have no problem using my pay-as-you-go plan. However, I had exactly your experience in China: I could only make emergency calls.
I love T-Mobile's approach, and the whole reason I use them is because they are the underdog who's doing things a little differently.