I posted this in the random talk thread, but I suppose the hard drives will snap horizontally into the top of the unit? That's pretty much the only way they could do it, unless they're planning on launching a line of "slim-only" hard drives. That, or the drives will all be built in and fixed.
How about this:
1)Slim drive only,but content is transferable,you get(buy) transfer kit...downsides-if you buy slim arcade,new slim drive required
2)Old drive fits,no slim drive...downsides(mostly for MS)-more expensive to produce console and drive,smaller profit
3)Slim drive,content is transferable,but old drive can be used(old drive is not incorporated into slim design,sticks out)...downsides-same as for fat 360
4)Any drive can be used...downsides-not going to happen
Personally i vote for 3