so FOX is wrong by roughly their margin of error, ie most statistical ties are probably in Kerry's favor slightly but may show Bush due to a slight bias. I think that that's reasonable.
I think I understand what you're saying
The only caveat is if your name is Scott Rasmussen. Then you would have no problem combining multiple polls over a long term period. Then, my poll averages can be viewed as composite polls with the entire sample size
That'd give me a real small MOE!
Seriously, though, Fox/ODs bias seems to be about 3%-ish in Bush's favor... maybe a little more (it's impossible to say which poll is precisely correct, but the poll median is around 24%, putting Fox/OD off of that by about 5%).