If I run a kennel, and I tell you that 60 percent of the dogs there like pork, while 70 percent of the cats there prefer chicken, you still don't know what the percentages are of dogs v. cats staying at the kennel. So I still don't have the internals of the IBD poll; thus I don't know if they're over or under-sampling Democrats.
+
Independents are +2. Obama is + 2. McCain two points does better with republicans than Barack does with democrats and gets 2 points more democrats than Obama does from republicans. I think we can infer from that that IBD is using a Dem +2 or 3 weighting.