I have preordered 4e and will play and run it this summer.
I anticipate liking the new edition based on what I have read so far, but I'm not going to make up my mind 100% until I have had the time to read, digest, play and GM 4e. By August I should know. If I don't like 4e, I probably won't be "staying" with 3.5, though. There are too many other good games out there that I already don't have time to play. I don't hate 3.5 but I ran it for years, two 3+ year campaigns, etc.
My guess is that the vast majority of folks will be won over in time because I believe the gameplay and prep times will be improved. I can't imagine someone who actually enjoys playing the new game more, deciding to stick with a game that is less enjoyable.
Someone can point me back to this post in August of '09, if I am wrong, but I really believe that the vast, vast majority of folks playing 3.5 will have joined the 4e ranks by then.
YMMV.