I'm hoping that at least one of the 3rd party publishers will continue to target and support the 3.5 market, which I think will still be around after 4E launches. 4E introduces an entirely new set of entry barriers for some players, and along with the other negatives clinging to WotC thanks to 3.5, magazine cancellations etc etc ... many customers are still irritated at them and are looking for a source for new 3.5 material.
I was hoping it would be Paizo since they are doing such an outstanding job, but I think that any company announcing their plans to continue supporting 3.5 gameplay, will gain some very loyal fans & customers.