John Gruber thinks this is total bullshit.
I do agree with John that at first this is way too early to even consider a larger variant, but if the iPad will be a success, the idea isn't too bad at all.
As a photographer I love the idea of taking the iPad with me to clients to check out and select the photos, present my work and even write an invoice on the spot. I'm already sold to the 9.7" version.
However, for serious editing work, I would love to see a larger version. And with larger, I mean LARGER, like 20-24 inch. Yes it wouldn't be portable, but look at it as a Wacom Cintiq with the added multitouch function and full computer possibility (the Cintiq is just a "screen"). Ideal for studio work. I can totally see myself working in Bibble 5 using a large iPad like that. And for on the road work, a version that is mentioned in the Techcrunch article of about 15.4" would be pretty nice as well. So the iPad becomes it's own product range, with an iPad (10"), iPad Pro (15") and iPad Studio (20") version. But since the iPad is running the iPhone OS, what would the larger variants run, it can't be iPhone OS with its limitations?
I guess "standard" OS X to make full use of their size and functionality. And thus this idea is almost immediately killed. Why would Apple support different OS-es in one product range? It would make support and development much harder.
It's much more plausible that the iPad will be extended with functionality, remaining similar sized (maybe grow a bit larger to max 12"). Still, I like the idea of editing photos/movies on a large iPad :-)