I used to have the grandfathered $5.99 data plan using epc.tmobile.com APN.
I recently changed my plan to the Even More Plus and had to sign up for the $10 data plan. Initially, I couldn't access the web or receive emails. Every time I tried to open a website, I was redirected to a t-mobile website saying I have to upgrade.
I popped my SIM card in a regular phone and web pages were loading with no problem. Then I put the SIM card back into my N900 and changed the APN settings to internet2.voicestream.com and everything was working again. I ran a speed test on speedtest.net and got the same down/up and latency as before (0.95Mbps/0.2Mbps and 100ms).

My conclusion is that with the $10 data plan, the APN has to be set to internet2.voicestream.com to be usable.