I have quite a few devices floating around, and only a handful of cellular lines to put them on. As a result, at any given time most of them are operating as WiFi-only devices. You should be able to use almost everything you need without issue, except for traditional cellular calls and SMS/MMS. For those, as pointed out above, one of the common solutions is to use Google Voice ? which uses data services for text messages and voicemails ? alongside an SIP (
Session Initiation Protocol) client such as
SIPdroid (here's the
Google Code site for it) or
GrooVe IP.
My primary personal device uses a $70/month pre-paid T-Mobile line, which I frequently don't pay and use my device as a WiFi-only device for that month. It all depends on where I'll be and what I'll be doing, but I've had nothing but good experiences so far.
Oh, and the battery life is phenomenal.
Edit: a word of warning: the Google Voice app isn't exactly the best thing out there.