I think it's perfectly acceptable for someone to make a purchase decision based on what some future product is supposedly capable of. It's called Future-proofing and it happens quite frequently.
If you're in the market for a new TV Today, you expect that TV to last you at least a good 5 to 10 years right? If you know that in the next year, or even the next two years you'll be buying some game device or some movie playing device, that can definitely factor into your decision as to which TV you buy. Knowing that the Xbox 360 will use component cables and output in 720p will lead you to want to buy a TV that can handle those things, likewise if you're planning on eventually getting a PS3 you'll want to buy a TV that supports 1080p and HDMI. If you're already in the market for a new TV these decisions typically only cost a little extra now, and ensure that you'll have the best possible experience down the road.
Game conferences and press releases happen to inform consumers of the products capabilities so that when the product finally does come out the consumer knows what to expect. Sometimes things can change, business decisions get made, and that's fine; but it's the product maker's responsibility to inform the consumer of these changes. I'm not talking legal responsibility, just a moral responsibility to the consumers, it's the right thing to do.
I think a lot of consumers and developers alike had an inkling that 1080p was too ambitious a resolution for most games to run at. But there are still a vast majority that aren't technically inclined enough to make that kind of assessment on their own, and so they must rely on the word of the product maker to deliver what they promise.
Whether you believe it or not these next gen consoles are driving a whole lot of HDTV sales, a lot more then HD Cable and the new Blue Laser formats have been. I can't remember the article but it was on the XS news page a few months back, citing that the launch of the Xbox 360 saw HDTV sales double and maintain a higher sales rate with people citing their reason for buying such a TV was to fully utilize their new console. I would imagine that the anticipation and eventual release of the PS3 will do the same thing, people who are in the market now are buying them planning on using it with their PS3 console and even more people will buy one after they get a PS3. You'd be a fool to think that you wouldn't keep the PS3 in mind when spacing out a new TV if you were buying one today.
Personally I'm still skeptical that GTV will even deliver at true 1080p. GT4 supposedly supported 1080i on the PS2, however it was really just faked, a higher resolution means nothing if it's being artificially pumped up to that resolution. PGR3 only renders at 1024x600 instead of true 720p, it looks pretty good but not as good as it could look, and I think the fact that it's not really 720p disappointed a whole lot of people. Another worry is that they might be demanding the developers deliver at that resolution, meaning other game graphics might suffer (lower res textures, lower polly count etc) so that the system can handle that resolutions. A number of Xbox 1 games supported 1080i but most of them looked like crap simply because the added resolution didn't make up for the low poly count and crappy textures. Here's hoping GTV TRULY delivers at 1080p without any kind of integral faking of the signal or without any sort of graphical loss to accomplish it.
Even if none of the games deliver at 1080p, Blue-Ray discs will, and you'll probably be able to upscale regular DVDs to 1080p as well. So if you're planing on buying your PS3 for BRD as well as the games then a 1080p would be worthwhile regardless of what the games render at.