GPS + PDA + MP3 = Walking Tour?

Over at Geeky Traveller, Fumio writes with this question:

I would like to know the practicality of PDA-GPS combo for walking tour, not for driving. For example, walking around Venice or Buenos Aires (European cities and South America). Where can I get such info?

That’s a darned fine question. I’ve blogged about a few MP3 walking tours (and I recently saw some on Audible), but nothing that combines GPS data. Good idea. Somebody get on that, eh?