They've updated the software recently to improve GPS accuracy and it seems to have done the trick - no random zig zags on my routes anymore!
You can pick up a lovely belkin armband for about £15, I just got a new one.
I don't use my iPhone as my phone anymore (moved over to an HTC android model) but have kept the phone itself as I love Runkeeper so much. As my phone has a cracked screen I'm not too bothered about dropping it or being mugged for it (well, obviously that would bother me, but its not a major concern). They have just released a version for Android but its only for v2.0 and above phones and only has the 'free' version features, but its promised they'll be improving it quickly.