Garmin's Mobile XT PDA software and Mapsource desktop software have this functionality, and the Garmin Mobile XT software is also quite fast in on-PDA routing. The problem for me with Garmin's Mobile XT software is the requirement for a Garmin GPS receiver. My HTC Advantage has a built-in GPS receiver which I'd prefer to use with GMXT but can't, TT Navigator works just fine however. And I also prefer TT Navigator's display of traffic issues in comparison to Garmin.
Now if I only had the ability to do trip planning and routing on the PC and transfer to PDA, I could use TomTom exclusively as my PDA GPS software.

Where as right now I have both on my PDA.
But if Garmin updates their PDA software to remove the Garmin GPS receiver requirement, and TomTom does nothing to add PC --> PDA routing then the Garmin software may just have to stay, even though I prefer TT Navigator's PDA functionality. Because I do a lot of motorcyle trip planning, the PC requirement is a must for me.