Fair enough, it gets semantic at this point, but I would say they probably, on a case by case basis, did something on the PC build, and then asked, "will this work on the 360 build" and tweaked things until it did. Tandem development yes, I'm sure I've heard that before as well, but the PC would have been the lead platform. So does that make the 360 version a PC port? Boy its hard to say for sure, my gut says yes, but the contrary argument is completely reasonable.
Personally, I'm of a mind that when Sony and MS say '10 year cycle' what they will really achieve is more like '2 year delay' before new hardware rolls. Move, Kinect, and shutter-lens-3D are stopgaps to stretch consumer interest in platforms that, if it weren't for the economic disaster the last few years have seen worldwide, would be in their last days as the lead platforms. Will this effect TESV? Will is be another 5 years of 7th gen systems, or just 2? These are questions without much to build an answer on at this point, but time will tell.
It is worth noting that the games that were developed for the SNES, PSX, PS2 and Xbox platforms five years into their life-cycles completely blew the doors off the launch software of those platforms. So a 7th gen console port of TESV could very well bodyslam Oblivion in the technical/graphics department, without the need for new hardware.
It's important to note, too, that technically the PS2 had a ten year cycle that overlapped the current generation. So we know that there's a precedent for a generation overlap.
Also, I don't know that the next generation will be a dramatic improvement in graphical quality. Look at PCs: any GPU over $200 simply isn't worth it, because the difference in quality between a $200 card and a $1,000 card probably isn't going to knock your socks off. I think we're reaching a plateau (at least in the short term) for graphical quality.