More mainstream? With HW-keyboard? Motorola Droid 4.
Under $130? Probably no.

But as "pocket-PC" as N900? lol, no!

PS.: You are asking for device better than N900 on TMO? There is no mobile phone (internet tablet ala pocket-tux) as good as N900. Maybe (a huge maybe) N950. But under 150 dolars?!