I tend to be a Skyper for my mobile VoIP, but I should point out the Gizmo Project if you’re looking for an alternative. Gizmo provides most of the same features as Skype, but can also natively record calls; comes in handy if you’re a podcaster of, oh…I don’t know…say mobile tech? ;)
The big news today out of the Gizmo shop is that the new version provides support for Asterisk PBX software, enabling some shops to route office PBX calls to Gizmo. Talk about mobility!