Last year after several months passed, I unexpectedly got back from them $1000+ with no explanation. I checked my calculations twice and had the entire return entered into one of those computah programs (post facto.) I checked it again before doing this year's return and got the same numbers (mine not IRS'!) Go figure!
I found information at http://www.irs.gov, but still no indication of whether this is an advance against 2001 taxes, or a straight refund. At
http://www.irs.gov/pub/irs-pdf/n1275.pdf you find the notice that was sent out to those getting refunds. Again, this does not provide any additional information.
At http://www.irs.gov/pub/irs-news/fs-01-09.pdf is a fact sheet which basically rehashes the same info. But they call it an 'advance payment', which leads me to believe that I was correct. I wish they'd just come out and say so. If anyone else can find explicit information stating that the $300/$600 will automatically be subtracted from your refund amount (or tax owed), I would appreciate it...