It's all a guessing game... sometimes with more or less information, or perhps based upon Canon's past practices.... For example...
Most likely, 1Ds MkIV will be introduced soon... rumor is that Canon plans to make some announcements in July or August, perhaps one or two cameras, maybe more likely two or three lenses. The 1Ds4 is overdue about 6 months, assuming Canon plans to stick with roughly the same upgrade cycle they've used with the previous 1Ds models.
There's also some speculation about what will be done to the 1Ds4... Most likely it will get a higher resolution sensor. Canon doesn't like not being the top dawg and both Nikon and Sony presently have higher resolution (24MP vs 21MP) FF cameras. It's anyone's guess... 1Ds4 might be 27MP, 32MP, 34MP, 37MP or something else. Besides that, it will probably have video capability and I would expect very high ISO capability (1Ds3 tops out at 3200 while the 5D2 goes all the way to 25600... that makes no sense except that the 5D2 came out a year later after a lot of ISO increases had been implemented in many models). Look to the 1D4 for other ideas what they might incorporate in the new 1Ds4.
There have been tons of rumors about 5D3... With high anticipation that we'll see one soon. But both previous models were intro'd about a year after their parallel 1Ds models... Some think to keep from undercutting the sales of the top-of-the-line camera. If they stick with this, 5D3 might not appear until Summer 2012, and then only if the 1Ds4 actually appears in the next month or so.
It's still a big "maybe". Canon can do whatever they want, whenever they want and once they feel the camera is ready to go to market.
The only really safe predictions are that folks will start posting here whining about the new camera long before it's actually available and they've had a chance to try it out, we'll see links to all the advance reviews (and can probably predict some reviewers who will love the new model, and others who will hate it), then it will gradually be accepted over the course of a couple months until eventually it's the best thing since sliced bread, then we will start to hear how new Nikon models are superior and people are thinking of "going to the Dark Side", then about 6 months after any new model is introduced the rumors will start about the next version and people will begin asking "should I wait for the 5D4" or whatever. Ultimately, it's far easier to predict what people will do here, than what Canon will do.