Fool Runnings - think you may be confusing the Nike Sportband (uses the foot sensor in the shoes) with the Nike Sportwatch (which uses GPS).
Wyn Thomas - I have the GPS one and it seems pretty accurate to me. Only thing I can think is that it supports both GPS and the Footpod and in one of the menus you can switch each on/off. It could be that you have GPS switched off and the footpod switched on (in which case you would need to calibrate as Fool Runnings says).
Only other thing I can think of is that when I connect to the PC it almost always downloads updates to the GPS settings. Maybe connect to the PC, see if it updates them and maybe this could solve it ?
I've been using the Sportwatch since it first came out. I had a Garmin before (405) but found it too complex and geeky. The Nike is far simpler, easier to use and I much prefer the website to Garmin's (where you view your training history etc.).
There's no need for the foot pod, unless you want to run indoors where there is no GPS signal.