First I must admit to having no personal experience using VoIP through mobile phones. Some/many networks also ban VoIP data connections or sell separate (Skype?) usage packages.
Pressing a bright (unless it autosenses usage) LCD screen against your oily face (when wireless headsets are inconvenient or unavailable)?
I'm not sure if Krisse is looking for positive or negative reinforcement to this idea, but certainly with the right accessories it could be done.
Stationary VoIP users probably outnumber mobile phone VoIP users by a healthy margin. Personally I'd also think the VoIP ecosystem would be better if it was done via standard protocols (SIP?) instead of a single corporation (like eBay's Skype).
Localized 3G data plans can also be murderous for those who frequently travel abroad.
If a somewhat N8x0-sized tablet was my only mobile voice device,