The differences between the PC (with loads of memory and a widely variable CPU/GPU setup) and either the 360 or PS3 is pretty significant, no matter that the 360 is slightly more similar, due to its threading model and unified memory.
The end result is that, by doing a port to either console, you've practically done the port to the other. Anyone who says otherwise doesn't really know console development very well.
Valve created a 360 version of their games... which is cool, but then, rather than "no comment" as to why there is no PS3 version, they choose to, over and over, knock the architecture, knock the online service, etc. None of these excuses really fly, and they're outrageously in the minority.
People like their games. They don't like their blame game, as to why there's no PS3 version. It makes them upset to hear one of their favorite devs say "sorry, its your system's fault that you can't play our games" when the truth is much more complicated, and so many other devs have leapt over those hurdles quite successfully, and without the blame-game commentary.