Originally, I thought this was an error:

I was wearing the new Nike+ Sportband, which has proved itself to be a great tool when running with friends because I can track my mileage without lugging my Nano around.

I have always learned the principal parts of 'prove' to be: prove, proved, proven. This would mean that the above sentence would need to say 'has proven.' However, a quick search on dictionary.com shows both forms as correct.

Life is all about learning.

Source: http://gadgets.boingboing.net/2009/08/26/my-nike-sportband-fe.html

http://dictionary.reference.com/browse/prove