Does Google Voice act as a SIP provider for SIP phones/software? (ie. can you use the N810's SIP capabilities with Google Voice?)

I wonder about using them with ZER01 (http://www.zer01mobile.com/). With the upcoming HSPA enabled Maemo 5 platform, ZER01 as your WWAN provider, and Google Voice as your voice/sms transport ... that could be your Maemo phone right there.

(or, if ZER01 ports their software to Linux and/or Maemo, you could skip Google Voice and just use ZER01 as your voice transport ... but I don't think they're doing SMS)