Yemenia, the national airline, offers plenty of internal flights in Yemen if you're in a hurry and can spare the cash (note that flights are considerably cheaper if booked in Yemen and not through an agency or over the internet. Book well in advance).

At the time of writing, a travel permit was required by all tourists when travelling to any place outside San'a and its immediate environs. Though travel permits are free, there's a catch. They are only issued to tourists travelling with a travel agency (Yemeni or foreign). In other words, independent travel is currently not on the cards. However, the ban should be lifted in the near future. When it is, buses go along all major asphalt roads - you need to buy a ticket beforehand at the terminal or at a major stop. Service taxis run on predetermined routes but without timetables - when they're full, they leave. Taxis travel most bus routes but also run on unpaved roads. You'll pay up to 50% more than you would for a bus. You can also hire private taxis. Rental cars generally come with their own drivers and are hired at travel agents.