I think if I were Paizo and I saw a rough 50-50 or 60-40 split (i.e. not a large majority either way), here's what I would do:
-Keep the Curse of the Crimson Throne 3.5 all the way through. When 4E is released, see if it's worth it to do some sort of free-PDF-download conversion guide for the last couple of issues in the path. My hunch is that it's doable, despite WOTC's efforts to discourage thought about such a thing.
-Offer the Second Darkness path in 4E. (This may not be completely feasible depending on how much of that path is already written.)
-By the end of the Second Darkness path in early 2009, it will have been long enough to tell how a) successful 4E has been in general and b) how the Paizo constituency is reacting. If 3.5 material is still selling well, consider an ongoing line of 3.5/OGL products specifically aimed at that group. Probably the best thing to do is use an existing brand name like GameMastery for that and use a new brand name for unconnected 32-page 4E modules.
No matter what the "I'm staying with 3.5 forever and ever and ever" crowd might say, the fact is that a sizable chunk of them are going to convert to 4E within 1-2 years of the system's release. It's hard for someone with X months' worth of 3.5 stuff ready to play to justify switching systems and "invalidating" all of that. As that material gets used, 4E will start to look like more of a possibility.
Pretty much the only way that "sizable chunk" doesn't happen is if 4E is a complete flop design-wise right out of the gate. Chances of that are fairly small, though. I agree with many that WOTC's pre-marketing has been sporadic and of questionable timing, but I don't think it's particularly in dispute that the design spirit is that far off. So if 4E fails I think it's going to be a marketing failure, not a design failure.
