TomTom may have had a leading position for sat nav and mapping on mobile devices but I predict things are going to change. Why? Google and Microsoft.
First the Microsoft Windows Mobile Team (well Jason Fuller to be exact) keep updating the really nice version of Virtual Earth Mobile - http://blogs.msdn.com/windowsmobile/archive/2006/09/15/757207.aspx - this does sat images, draggable maps, traffic, gps integration, driving directions and more. Just a shame they haven't offered more support for this and made it an official release. It also a little unstable.
Now Google have released Google Maps for Windows Mobile - very similar to the MS Offering, though the UK sat map coverage is not as good. However it does appear more robust. Get it from http://www.google.com/gmm/
Having sat images on these devices is really good, and a real help in navigation, though data traffic costs is something that needs to be considered; existing mobile navigation providers will need to address it or face a big challenge from these players in the future.
Ian