Both systems have great equipment and there is no way you can switch pro platforms without losing a couple thousand dollars in the process. Sure, currently the D3 has less noise at 6400 ISO than Canon, but it's not worth losing money over. I'm sure Canon will respond soon enough.
If you're a sane pro, it's either an ROI decision or it's a capability decision. For instance, if you need publishable results at ISO 9600, you've currently got one camera choice that'll make the shot- the Nikon D3. If you don't want a one-off, then it might not be worth losing the shot- or you might need tilt/shift lenses that Canon offers and not be willing to move up to LF. How many low-light opportunities will you miss between now and the next Canon release? Six months worth? If they're lucrative that could pay for the switch right there.
Besides, people switching are a great source of well-priced, well-cared for gear, don't knock them too hard! I saved almost $2000 on one lens because someone was switching over to Canon because they felt they needed the difference in high-ISO performance and "look" at the time.
Heck, I've got a product shoot next week, and I'm totally bummed that I didn't get some new monoblocks last week- I'll shoot them on location with my current lights, but it won't be nearly as easy as it would be with the new lights, so I'll be sinking more time into a project that I'm pushing into my schedule that's filled enough that I might actually have been able to ROI half the cost of the new lights in one shoot.
If I needed something Canon had, and I could ROI the switch- then if renting didn't make sense or wasn't an option- I'd switch to get the tools I needed.
Too many people make the tool choice a religious one.
With that said, I often find it amusing that "pro" shooters on DPR switch back and forth when it's obviously their lack of skill or knowledge that's the issue. But again, it's a great source of gently used gear at great prices, so I say let 'em switch their hearts out!
