If it's supported by the sort of games I like and if it has a special action that lets me drop my pants and moon the big enemy boss, then I MIGHT think about getting it

There's actually the Emotiv if you want to wear a helmet thing so you can SHOOT PEOPLE WITH YOUR MIND but I don't know how well it works, and it's $150. Jesus.

eeeeh, pass. I've looked at motion controllers in the past and not used them for more than a 5 minute gimmick. I just want to lean back in the couch/chair/beanbag and relax, not jump and flail around the room.

Fable III supports Natal in some manner and is coming to the PC, so it's a foregone conclusion that the PC version will have Natal support as well since Microsoft will want to push it on the PC if they can.
Microsoft will be releasing the SDK as soon as they can to get developers working with it, but as with the Sixaxis controller's tilt functionality it's entirely up to the developers to actually make use of it; since 360 Natal code will be portable to the PC in a couple of clicks I'd imagine any game incorporating it will do so on both systems. It will be much harder to integrate for non-gaming purposes because I imagine only the most basic of gestures will work on third-party software, and those that use non-native windowing elements (like Steam) probably won't respond as expected to such commands.
Another gaming concern is that, as with the Wii, developers won't always implement the motion controls correctly. Unless Microsoft has some catch-all templates to look for I can certainly foresee the Wii issue returning of games seeming to decide almost randomly whether to accept a movement as the correct one or not.
That being said I can't see myself paying for a peripheral to flail around, whether for a game or the OS itself; it's something I'd really need to see in action, and the rumoured launch price isn't too attractive either. If it can double as a normal webcam or something that would add some value (although not for me), but either way I can see this selling more to those with 360s than those with just PCs.

Imho, the only interesting thing about Natal is the ability to control a multimedia interface from thin air. Gaming is a serious business, the so called "augmented" interfaces like touch screens or cam-driven controls can go fuck themselves right now and forever :-)