Not sure how well this will work for older blackberries but the 8900 APN for T-Mobile is epc.tmobile.com
Supposed to have better page rendering for newer berries and it seems to be a bit faster.
Check that link for all T-Mobile APN settings. The sidekick setting works on all data phones now and the 'G1' APN is actually the new standard for all new blackberries and other smart phones from T-Mobile.
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