Ditto on what bertimus2002 said.
With a voice, plan:
$19.99/month = unlimited BIS
$29.99/month = unlimited BES
Without a voice plan, add $10/month.
To the OP, I really hope that T-Mobile won't raise their BlackBerry price plans. For business folks on the BES plan, a $10 price increase would put T-Mobile's rates within $6 of Cingular/Verizon, which greatly reduces the "get more" effect.