Why does that depend on HS*PA working yes/no. Based on the specifications of Maemo 5 and RX-51 there are many other reasons a United States resident would consider to buy such device.

I don't know about VoIP given people seem to relate it to Skype and other unnecessary proprietary technologies but regarding SIP: you can use a SIP server with or without PSTN support. You can use this by default on devices which support TCP/IP and internet connectivity. For example a laptop, a mobile phone with data capability, a RX-51 with data capability, or your using WLAN in your friends house.

You could opt to either use your cellphone with GSM + data (probably a so-called smartphone) or you use the RX-51. Or you toss in the SIM card in your RX-51. You can even log in several times on one SIP server allowing you to phone with either logged in client, but only the last logged in client will be able to receive an incoming call.

For now, sure. That has been announced and has been clear for a long, long time. This is Harmattan; long term; think post RX-51, post 2009.

So, for now, you could combine that with a simple Nokia phone for GSM (prepaid or subscription). That wouldn't cost much, so it'd allow more budget for the tablet and the data-only subscription.

Also, this model is not for Maemo only (Intel does not use Maemo, but uses Hildon). It is probably also useful for Moblin. And maybe even S60 (Qt has DBus support).