Oh I don't think so. Google Voice is VoIP.
And, depending on your mobile coverage, call quality may be very poor - especially when you're in EDGE-mode, it is actually not possible to reliably use VoIP.
All Nokia phones support a second phone line. Nokia should really add this capabilty to the N900. I'd really like to buy this phone, but if it is not working, I'd have to wait for its successor - that's a pitty!
I there active work beeing done on the phone app, or is that closed source? According to the CPHS specifications, ALS support only requires the phone to send a different bearer to initiate calls on the second line. Plus, the phone has to distinguish incomming calls. Someone should hack that into the phone app. Should be a minor fix, I guess...