I'm running two monthly games, one 4e, and one 3.5e, as well as a 4e game at work during lunch breaks.
The 3.5e one has been going on for two years (we are level 10, gestalt), and is more like 3.75e now. I use 4e stuff behind the scenes, to make monsters more interesting and easier to create, and am starting to use it for the NPCs too. The latest big, plot-quest items also have a 4e feel to them, having at-will, encounter and daily powers. And, some items the party have have also been 4e-ed. (a knockback bow, normally 3/day, has been changed to 1/encounter; a set of armour that casts 'blur' 3 or 5/day is now 1/encounter).
I won't admit to it being more than 3.75 until the horrid BAB / iterative attacks and the crazy number of spells for casters are both gone
I also run some games online, with 3 out of the 8 being 3.5e. These, too, have been going on for some time, and I would be happy to convert them to 4e, if only the players would all agree

Better to keep players than to change editions. For the moment.
Book-wize, I allow pretty much all WotC books, as long as people don't go crazy building seriously over-powered characters. The Book of Nine Swords is one I happily exclude, apart from games where it is already accepted that characters are going to be over-powered, as it is a significant step up from the general playing field.