I'm pretty sure the 360 version was developed simultaneously with the PC version. I remember Todd saying something about that. The PS3 was ported. I could be wrong, but this is what I remember.
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.