mentioning a part-time job?

So I understand how I might seem a little ridiculous here, but I've actually been asked about my GPA, so yeah:

I've been working part-time ~15-25 hours a week throughout college to help pay on top of my other stuff, so my GPA apparently isn't that great to my interviewers (barely over 3.5). A few guys have asked me about this, so I'm sure other interviewers have thought about it before.

Although he's a terrible example, jeffrey chiang slipped in a line about how he worked to pay off college. I doubt it deserves a line in the resume, so should I include it in the cover letter? But then again, some people don't even read the cover letter, so is it worthwhile to put it in the resume? Any advice?

Thanks a bunch,

BankingWaffle