If you make and receive very few calls, as I do, you might consider the Email and Internet only plan. If you look closely at it, you will see that it isn't really restricted for Email and Internet only, but phone calls are allowed -- they are just pretty expensive per call.

You can find descriptions of the plans on the Tmobile site.

I ended up going with the contractless unlimited data only plan for $33/mo. (that includes a military discount). All of my voice is VoIP (even now that's the case as I use AxVoice combined with Google Voice). Looking forward to it, and thanks everyone for all the advice. I'll let you know how it goes.

Mobile phones can still make 911 calls even without SIM cards, so you shouldn't need to worry about that. You still have the problem of sometimes not having a good enough data connection for VoIP calls though.