Yep, sure is. In fact, that's for the most part how I've been using my phone(s) for the past several months. I don't carry a voice plan, or if I do it's just prepaid (nice to have just in case). I use Google Voice + Sipdroid. Personally, I don't talk on the phone much anyway, so the solution works. Right now I'm actually trying the iPad data plan on my phone... unfortunately while 1900MHz UMTS (3g) on AT&T is available here in Huntsville, I'm barely on the edge of the tower's range it seems while I'm at work, and I get zero coverage at home on it, which was a hope (I'm on a T-Mo Vibrant, so I can only get 3g on the 1900MHz band on AT&T).
But like you, I'm near wifi more often than not, so making calls is still pretty easy to do.
BTW, an iPod Touch won't work - you'll need a separate mic. Buying a cheap old iPhone is a better deal... plus if you end up with a 3g or better you can hop on the iPad data plan and get voip even when you're not around wifi. I did the math, and for $25/mo that 2GB equates to roughly 3000 voice minutes using VOIP (minus any bandwidth you use for data during the course of the month of course).