So one week on, how did it pan out? In short, not bad... not bad at all.
We had about 100 users come on board on Day 1. We saw a reasonable amount of load on the TFS servers for about 4 hours, as users populated their workspaces. After that (and subsequent days), things tapered off quite nicely. We regularly see 50% spikes on the DT CPU, but these generally don't last more than a second or two. The AT seems to basically have its feet on the table.
That said, we did run into one hitch with the MSSCCI provider. We found that creating a new branch doesn't cause the provider to prompt for binding information. It simply sticks with the original, which is definitely not what we want. There is a workaround (namely to rip the binding info out of the branched project files etc), but it ain't pretty. I'm sure this will be sorted in a future release.
Looking back, we had a comparatively smooth ride, even though we targetted VS2003 users. For anyone considering such an upgrade, I'd recommend you go for it. Just remember to plan and practice everything.