I picked up a Vertical Communications 7504-10 IP phone from a yard sale a little while ago on the assumption that since it was standards-based (SIP) that I'd be able to use it with Asterisk. Browsing around with google only revealed one thread where someone claimed that you could make the phones work, but said nothing about getting into the phone in the first place. But as it turns out, it's actually quite easy to configure these phones to work with an arbitrary SIP provider.
I've got an Android phone with, currently, no phone service. Well, OK, it's a WinMo phone chainloading Android, but that's relatively irrelevant. Soon I will want to avoid using minutes from my incoming T-Mo PAYG phone card, so I'll still want to make phone calls using SIP any time I'm in WiFi range. You can even get out of paying for outgoing calls so long as you turn them into incoming calls.