@aldvan: I'm not exactly sure of your situation, but if you absolutely need an second camera body, why not just get another 5D2? You can always get a used/refurb body and sell it once the 5D3 comes out... After all, you already mentioned that ur happy with the current machine.
I admit that this was one of my first ideas. Same command layout, total interchangeability. The plus of a 1Ds is in a way related to the better AF, not for the AF itself, but for the ability to manage the 1.4 extender on my 100-400, opposed to the 5D's...
Yeah, that really can be a tough choice.
OK, let me lay out the alternative bodies here:
1. rid 7D, get 1Ds3
2. rid 7D, get another 5D2
3. hold onto 7D, wait for 5D3 (or 1Ds4)
So u've ruled out #2, leaving you with #1 (which leaves you at a loss of nearly 4 grands for an older body) or #3 (putting ur trust with the fickle finger of Fate). Truthfully speaking, neither sounds good.
Well, if you can wait for a while before making ur choice, why not rent the 1Ds3 for a weekend and see if you like it? That experience should give you a better idea of whether you want the 1Ds3 (You might even want to try out the 1D4, since that's the model with current top-of-the-line tech).