It depends really. It depends on how the weight of the monitor would be distributed across the case of the 360, and how much weight there would be per square inch on the case.

Personally, I would say 10lbs is probably a bit much to be putting on the 360. I've put close to that much on a "fat" PlayStation 2 before without problems, but the weight was distributed across almost the entire case, and it was a hard plastic, not the cheap flimsy crap the 360 is made out of. I generally follow the advice of others, and make sure that the 360 has about 6" of clearance on all sides for proper ventilation. Microsoft cut quite a few corners in the design of the 360, and so it generates way more heat than the case can reasonably dissipate without some additional help.