T-Mobile now lists the BB 8900 on their site. You can't buy it, yet, but it looks like it'll be available in the not-as-distant-future as originally expected.
http://www.t-mobile.com/shop/phones/Cell-Phone-Detail.aspx?cell-phone=BlackBerry-Curve-8900
http://www.t-mobile.com/shop/phones/Cell-Phone-Detail.aspx?cell-phone=BlackBerry-Curve-8900