I'm curious: do you have a data-only plan so you can use VoIP to make phone calls?

I have been trying to find a device that would make it easy to make VoIP calls most of the time, since I am nearly always around free (or already-paid for) wireless broadband, but fall back on a prepaid SIM card when I only have cell network coverage. Some of the Android devices look promising for this, especially the Nexus One, but I have been unable to find hard information about whether it's possible to do this conveniently.