The old Side Kicks and services are being phased out. There is a new Side Kick 4G coming out, and sounds like it will have other service options.
Signup page for more info
http://sidekick.t-mobile.com/
review
http://www.pcmag.com/article2/0,2817,2376019,00.asp
unconfirmed specs
http://www.phonearena.com/phones/T-Mobile-Sidekick-4G_id5158
http://www.slashgear.com/t-mobile-sidekick-4g-leaks-with-samsung-branding-28128987/