I'm running 2 D&D groups and play in 1, plus sometimes take over for a neighborhood DM when she's on night shift. We have custom rules implemented, based on 3.5 but heavily modified - complete with custom classes and races and feats and spells etc. Dragons also got changed already, coming close to what 4e is going to have. Asides, we've added stuff from GURPS and other sources. My hubby and me have a laaaaarge library including self written stuff. Plus, hubby programmed an auto-update Excel map for it all, so we aren't switching. Anything we like in 4e, we can implement without having any backwards compatibility issues.
As for the mood of the players...
Group 1 I'm running, 2 players - we also created a custom world with some weird extra house rules. My son (16) is in this group. We aren't switching, and wouldn't even if we'd not have such an extensive library in 3.5 because he's learning disabled and always takes long to remember rules. The other player (son's friend) does what son wants.
Group 2 I run couldn't switch if we wanted to - 13 players and me as DM, just done with season 2 of an Epic campaign with at least 13 seasons involving 13 worlds. We took more than 6 months to where we are now and that was only because during the Xmas holidays, we had our private convention and played nonstop.

You can imagine how long it'll take (and yeah the group will survive, we did this twice before) before 4e would matter.
From those players, only 2 showed a vague interest in 4e, but quickly saw that we can adapt the few things we actually do like from there. 8 are outright against it and would probably quit. 3 others wouldn't care either way once the campaign is through.
The group I play in, 4 players and the DM, we just made our own world and some house rules and play 3.5. As we only get together once or twice are week, things are proceeding slow so there was no talk of 4e yet. But knowing the DM he'll stick to what we already have. I'd not mind trying it if he'd switch though.
The group of the DM I sometimes help out (5-9 players depending who can make it) doesn't care at all but does not want to make new characters, except for 2 of them. That's why they are happy with 3.5. Plus, of course, the DM isn't switching because she uses our Excel sheet

Right now they are playing CoC though so who knows what they will really do afterwards.
I also run a SW RPG (7 + 5 players in 2 groups, but still same campaign) . The players expressed interest in trying D&D to do something different once this is over. They would all be ok with 4e and I think some of them might even buy the material. In this case I'd DM for them if they'd get me the books so I'd not have to spend more money.