First I will say that most multiplat pc,ps3, and 360, are made on pc then ported to the consoles. I will also like to say that there are alot of developers on the ps360 that like to push graphics areas and usually were on pc previous gens. Now once pc starts to set itself apart from the consoles graphically,and these developers such as epic go back to the pc , and the ports start becoming less and less what is going to happen to the ps360s  3rd party support?( there are alot of pc/ps3/360 multiplats) Also what will be the main developement platform? ( I'm guessing it will split between all three consoles).

Developers won't push PC graphics to the point where they can't even make a downgraded port to the consoles. Consoles are a quick buck for these companies.

Well I think a lot of devs will wait till the new X360 comes out before they developp another Super visual beautifull game so they also can port it to x360 and have more income. Look to Crysis;..It is very tricky to develop a game that will be stronger than Crysis.....Costs a lot of money and have a big chance to sell not that well.

We might see less titles on both the Xbox 360 and PC. But I don't think there will be that much of a difference. When it comes to high budget games, the Xbox360/PS3 is far harder to ignore than the PC.

 Well we are already seeing pc games pushing the hd consoles to their limit. See farcry 2. That game runs pretty bad on the hd consoles compared to pc, and there will be more and more games like that. 

 Ok here are the top rated games for 360.

http://www.gamerankings.com/itemrankings/simpleratings.asp

Title	Plat	Company	Reviews	Avg. Vote	Avg Score
1. Grand Theft Auto IV	X360	Rockstar Games	71	8.3	97.230%
2. The Orange Box	X360	EA Games	56	9.1	96.300%
3. BioShock	X360	2K Games	93	9.3	95.387%
4. Call of Duty 4: Modern Warfare	X360	Activision	77	9.1	94.155%
5. The Elder Scrolls IV: Oblivion	X360	2K Games	105	9.2	93.840%
6. Gears of War	X360	Microsoft Game Studios	112	8.7	93.750%
7. Halo 3	X360	Microsoft Game Studios	89	8.5	93.442%
8. Rock Band	X360	MTV Games	57	7.0	92.727%
9. Guitar Hero II	X360	RedOctane	62	8.1	92.253%
10. Mass Effect	X360	Microsoft Game Studios	80	9.2	9

 2 Pc port

3 Pc port

4 Pc port

5 Pc port

6 Shows of unreal engine 3. ( most games like this would stay on pc so they could show off new engines)

That is 6/10 games we wouldn't have if the pc set itself apart. 

Now for the ps3. 

Title	Plat	Company	Reviews	Avg. Vote	Avg Score
1. Grand Theft Auto IV	PS3	Rockstar Games	57	7.9	97.286%
2. Call of Duty 4: Modern Warfare	PS3	Activision	45	7.8	93.682%
3. The Elder Scrolls IV: Oblivion	PS3	Bethesda Softworks	56	7.7	93.054%
4. Rock Band	PS3	MTV Games	29	--	91.490%
5. Uncharted: Drake's Fortune	PS3	SCEA	72	9.0	89.812%
6. Ratchet & Clank Future: Tools of Destruction	PS3	SCEA	73	8.3	88.497%
7. Burnout Paradise	PS3	Electronic Arts	47	9.0	87.904%
8. The Orange Box	PS3	EA Games	21	8.9	87.881%
9. Resistance: Fall of Man	PS3	SCEA	79	7.9	86.782%
10. Ninja Gaiden Sigma	PS3	Tecmo	53	8.7	86.542%
Totals:   10 Titles

2,3,8 are pc ports. Other than that the ps3 wouldn't seem to take much damage, I but it wouldn't be as good as it is now. In conclusion, the ps3 and even more so the xbox 360 rely alot on ports of pc games. 

To find out what happens look at some of the previous generations.