I'm on the Sidekick plan right now on my N900. It works normally. As mentioned on my numbered message a ways back, calling Tmobile is not what you should do about this kind of problem.

Edit: By the way, I was on wireless when I switched my plan from Pay by the Day to Sidekick and instantly got a text message confirming that the plan change had happened. That's nice.

Here's some other information. A reminder about APNs. The APN I'm using for this Sidekick thing is epc.tmobile.com.

But another thing I have played around with lately is the APN wap.voicestream.com.

It allows you to do some Internet things without an Internet plan at all. I could, for example, use m.yahoo.com and read my email. There's a recent thread about that on this site -- I think the header mentions prepaid tmobile like this one does.