iPhone compass broken?

I've written a small application for using the iPhone's 3GS-and-better magnet. It is loosely based on Apple's Teslameter sample code.

What I've noticed is that on some iPhone devices it works. O开发者_开发问答n others, there's a first exceptional reading - and then, no more readings.

What's common to those problematic devices is that they were exposed to magnets longer that your average iPhone...

So, I'm wondering if (1) could this have "broken" the iPhone compass? (2) if yes, is there anyway to fix it? (3) is there any way to "reset" via software?

Thanks!