1) Most of the carriers have terms of service agreements against that kinda thing, yeah. But that's just a matter of "if they catch you".

2) quality of VOIP calls is frequently not as good as quality of circuit switched calls. Even with ideal signal strength (even on a wired network).

3) for skype at least, SMS to non-skype users is one way out to them, no ability for them to reply. That makes skype significantly less useful to people who SMS a lot (like I do). And it doesn't help that skype uses a proprietary mechanism for txt messaging. They should at least have an option for: a) using a jabber client for the user's interface, and b) 2 way support for SMS and MMS.

4) you'd have to stay logged in to skype to get things like voice mail and missed call notifications. Which is sort of 50/50 in a way -- being able to turn off skype/etc. so that you're not getting calls, and the calls go straight to that service's voice mail, but still use the data capabilities on the device is better, IMO, than silent/vibrate mode in _some_ situations (maybe even most, but not all).

5) right now, the tablets don't have silent/vibrate mode. And even if they did, the VOIP clients would have to add support for triggering your vibrate mechanism.