My theory is that the upgrades are a necessary precursor for the upcoming 1Ds Mk IV with a sensor so packed with pixels that it would outresolve current lenses.
Someone else raised essentially this same theory. Actually, I'm inclined to believe there is something to this logic. I think the previous individual suggested that since lenses are often on a 10-year cycle, it makes sense for Canon to start upgrading their L lenses for the next generation of sensors.
In my opinion, that's the most logical explanation for the new 70-300mm f4-5.6, which seemed overpriced for the current market but not necessarily so if we see a significant boost in resolving power over the next several years. Of course, I haven't seen any tests of this particular lens to determine just how good it's resolution may be, but I'm guessing that all the new L lenses will see a big boost in resolving power.
For old guys like me, this is a concept that's hard to get my head around. I grew up with film and the old rule was that even the cheapest lenses could always out-resolve the best film. Clearly, that's no longer the case with digital.
Regardless of what model camera body Canon releases next, I think the march toward ever-more pixels is going to continue, probably limited more by practical file size than sensors. I'm still of a mind that at some point they will be releasing a camera that allows for digital zoom in the body, along with optical zoom in the lens.