I've not done any real work on this so far, but here are some of the web hits which look useful:
- C# to Nike+ API discussion - http://www.wroolie.co.uk/?p=13
- Some simple XML - an example of the public run API - no idea who this user is!
- ear fung us - a wordpress plugin
- a php wrapper for the api - http://lerdorf.com/php/nikeplus.phps
- this site seems to be based on n+ - http://www.workoutdump.com/
- yet more discussions and examples at http://www.shokk.com/blog/articles/2007/01/10/nikeplus-api/ , and more besides