I've run a 4E session tonight, using the information gathered here on ENworld and from about the net. We used the Level 1 characters from Experience on the elven Rogue from these boards.
The kicker for my players is that what happens in this play session with these characters happens in my campaign world we'll be playing in on June with the release of 4E.
And they were all happy and looking forward to it.
I'll put up my review of the experience later.
To answer the question. No. 3.5 will go away, and that's fine with me. I got burned out last summer, and we took a break from it in general and started playing other games, from Mutants & Masterminds to Warhammer Fantasy Roleplay 2nd Edition. (Even flirted with ShadowRun 4th Edition, but that system just hates itself and tries to commit suicide with unnecessary complications.)
And while playing, those other games, I realized that I had hit a saturation point with 3.5. It was no longer fun for me to run as a DM. All the extra work as a DM, was being spent on the mechanical aspects of opponents. And what not. Which then caused me to pick up a module against some hobgob's and tiamat avatars.. which is really interesting, and will be fun to convert to 4E.
But 3.5 it's going to the shelf and there it will stay. Though some materials/items such as the Ptolus campaign setting will be adapted to my world, and adapted for 4E.
But, the curve in the game with save or die mechanics aggravated me, and it was too prevalent in 3.5 still.
So, it's gone and my time off from 3.5 and D&D in general is just fine, I'm rejuvinated in ways that I didn't realize I had been drained off completely.
For me, AD&D 2nd Edition was fun, but when 3rd edition came along, I jumped right on it and learned, and was a little pissed with 3.5 coming along, but the whole 3rd/3.5 editions were good for their time. But 4E has come at the right time for me. Though I still think it was 2 years too early, with the money sink known as 3.5. Seriously.
But our group will adapt and as far as the books coming along, I'll get the core stuff, the FRCS, and other books that expand on options for characters ala Martial Power. And other such books. Beyond that, I'll weigh each book on its merits and take it or leave it as necessary.