I just got my $300 rebate for VS.NET. I was a bit worried because the rebate sheet said that the only valid proof of purchase that would be accepted for VC++ 6.0 was the original box-top (which I no longer have). Just to be hopeful, I sent in the cover page of the old manual (which was what they used to consider Proof of Purchase), and they accepted it.
That's less than a month between sending in the rebate certificate and getting the check. Not bad.