Having each app on a separate device would actually make it worlds easier to perform real work, because I'm no longer Alt-Tabbing or Expose-ing back-n-forth between programs. I hate doing that because it forces me to stop thinking about my work in order to think about how to manipulate boxes of pixels into the configuration I want.
Right now I'm considering spending another $1K to get a 27" Cinema Display to go next to my $2k 27" iMac. But I'm wondering if I should have spent that $3k on a stack of wireless tablet computers of various sizes and leave some Bluetooth keyboards around the house instead.
Edit: Now that I'm thinking about it more, I believe this is the trend and the reason desktop innovation has stalled. In a decade we'll all have solid state hard drives and ten times more RAM, so every app can run in its own VM. You could suspend an app's runtime state to disk, transfer it to another device and resume where you left off. "Bump" your Final Cut session to your big-screen when you need to, then "Bump" it back to a tablet when you wanna continue editing on a park bench.
I don't know why we can't do this with web apps already. It transcends the whole idea of a desktop manager.
Nor is there anything impractical about process migration. I suspend and resume Windows VMs all the time, and Windows wasn't designed for that. Think of what's possible when the programming language and OS API makes it easier to write programs that can re-orient themselves.
And even if process-migration doesn't become a feature, there is still the inherent advantage of manipulating "windows" in physical space. It's more intuitive, it's more convenient, and it's direct. When people didn't "get" the iPad and dismissed it as just a big iPod Touch, it's because they didn't grasp the benefit of manipulating the UI directly instead of through a mouse.
When you manipulate windows with a desktop manager, you're two steps abstracted: you use the mouse to manipulate the widget that manipulates the windows. THAT is what's impractical.