Actually--Talks (and Mobile Speak) only work on GSM providers; in this case, Cingular/AT&T or T-Mobile.
I use Mobile Speak and love it.
An option for Sprint may well be the Motorola Q, which is made
accessible with the Mobile Speak Smartphone software, which runs under
Windows Mobile for Smartphones. If you want all the functionality of
your phone (contacts, SMS, Email, Web, etc.), this is the way to
go. For basic functionality, one of the LG phones should do.