This won't be used for cross-platform games, because Sony isn't going to bother to set up a server farm for this. MS can do it cheaply because those 300,000 servers are just part of their Azure platform they've set up for businesses. They'll sell all the idle server time for business use like Amazon does.
MS might pay EA to put in BS code to use the awesome server power to add puffy clouds or better trees or something (shades of hardware PhysX for extra sparks or dirt), but nothing that matters. Or like the games that crippled the DX 9 graphics to make the DX 10 graphics look better by comparison. ("These shadows required Vista and DX 10, no way we could do them in DX 9, nuh-uh!")
Randomized worlds have been done locally without servers since the DOS days, and probably since the Apple II.
Servers make sense for multiplayer, but for single-player I wonder whether it will ever make sense. If the server is generating hundreds of MB of data, it still needs to be transferred to the Xbox -- will that really be faster than generating it locally? If there isn't much data, then was it a big enough job to be worth sending off to the server?