It would be silly to try and put an open world game on a single disc. Mass effect was not an open world, thus could be split into more then one disc. I havn't been keeping up with RAGE enough to know how linear it is but apparently enough so that they can split it as well.
And lets not forget that the XBox DVDs can hold ~9.7GB of compressed data. where as oblivion and Fallout 3 were only ~5GB on disc
Ya, there was an update that made the Xbox discs hold more data and the update was when PSN was down.
If it needed to be on more than one disc, it would be on more than one disc. Therefore, it doesn't need to be on more than one disc and you don't have anything to worry about.