There's Viewranger, Memorymap and Memorymap Free, UK OS, which doesn't have route tracking but gives you free 1:10k mapping for the whole UK. Anquet are developing an app for the iPhone as well. iPhone 4 really is the one to go for; more RAM, faster processor, stronger case. I cracked the case on my 3G by dropping it on the headphone plug, the 4G socket goes straight into a steel frame. There is a handlebar mount made by Dahon which is weatherproof. Reviews have said that it can pop open on rough ground, but a thick rubber band keeps it closed, and it's not too expensive, IIRC.
I've got Viewranger on mine, and as I already had the full set of 1:50k maps for my old N95, it's easy to copy across any maps via wifi, then you can add 1:25k maps OTA, plus European and US maps as well. Panoramic display for Viewranger has yet to be added, but updates are coming all the time.
Edit Here's a nifty mount I found, a bit of clingfilm over the phone and wrapped underneath if it rains would keep the weather out.
http://www.fommy.co.uk/view-full-page.asp?skuno=88677
Correction, it's UK Map for the free 1:10k mapping, and there's Motion GPS as well.