Take a look at ridermount.com, there's a guy selling waterproof iPhone mounts with a m/c specific fitting, you can also get a ram ball set up. I have the Tom Tom app on an iPhone 4, it's accurate and extremely clear but unlike garmins there's no way to plan a detailed route. The gps chip is also power hungry so you will need to power it from the bike or it won't last long. Went camping in Cornwall a few weeks ago and with the Tom Tom on shortest route it took us down some cracking lanes, over fords and through villages well off the beaten track.
I have OS maps down to 1:25k but they don't include land ranger detail so byways/rupps etc are not marked