Republicans want Brown seated ASAP, if he wins. Democrats, naturally, would like to stretch out the process as long as reasonably possible. I think that if Scott Brown wins tomorrow, we'll be hearing a lot of back and forth on these precedents. My summary: the folks in power tend to do whatever they want. Which is usually okay, because it just doesn't matter that much.
I don't know which narrative will catch on. But I'm not sure that's the most important factor. Whether or not it's fair for Democrats to stall Brown's swearing-in, can they actually do it? Or rather, can they actually do it, and then pass a massively unpopular health care bill on a straight party-line vote? Even if only a small minority think of this as illegitimate procedural shenanigans, that's going to further tarnish the chances of a party that's already under fire for cutting too many DC insider deals.
That's not to say that Democrats won't do it, of course. The Democratic determination to commit electoral suicide over this issue has been impressively firm over months of negotiations; there's no particular reason they should stop now.
Either way, though, we can look forward to at least several weeks of Bush v. Gore style continuous arguments over previously unimportant procedural points. I suppose it's a nice break from the death panels and the accusations of mass murder.