I said something like this in another thread (forget where at the moment), but the logic was similar. By putting the iSight into the iMac line, Apple is pushing the iSight into a far larger percentage of their user base. By doing so they are opening the way for new iSight apps (like Photo Booth) and adding a little pressure to those who haven't gone to a high-speed internet connection (it is the only way to use the high end iChat AV features). Adding a little to the high-speed internet aids in the spread of iPod video downloads, which also need a good internet connection. The newly expanded HD movie trailer gallery with its 100MB+ files also adds pressure.
Anyway, to aid in the spread of iSight, they need to get it into the Pro desktops as well, and the only way to guarantee that is to build it into the Apple Displays. For some who have multiple displays, though, having 2-3 iSight cameras aimed randomly around the room may not be a great idea. Therefore I predict that the new Displays will have a lens cover (something sorely lacking on the iMac). So they don't leave the portables behind, a new, smaller iSight camera will have to be released too. Hopefully the camera will have a higher resolution, better color reproduction, and faster capture rate. Being able to better sit at the top of my PowerBook's screen would also be nice. the current model is a little bulky in that respect.
On another note, although it is highly unlikely, I'd really like to see a 17" Apple Cinema Display (w/ or w/out iSight). I can dream...