Here's my take on the whole Porsche (and etc.) thing for the game.
As much as you guys want to pretend, RUF does not make up for Porsche, here is why. Let's compare the models available in Forza and the ones in GT4.
Porsche model in Forza - GT4 equivalent's through RUF
Boxster S = 3400S
911 Turbo 3.3 = BTR
996 GT3 = RGT
993 GT2 = CTR2
959 .. CTR "Yellowbird"
911 Carrera RS '73 ...
Carrera GT ...
911 GT1 LMP ...
550 A Spyder ..
Dauer 962C ...
944 Turbo ...
911 GT3 Cup..
911 GT3-RS..
As you can see, there's a lot of Porsches that the RUF models in-game do not make up for. Let's say I don't really care that much for the 911 '73 since we got the 911 Turbo 3.3, but what about the 959? Does the CTR really make up for the 959 which is an AWD. I'd rather have a GT2/CTR2 and a 959 instead of a CTR, especially if a 911 Turbo 3.3 is already in there. What about the Carrera GT, 911 GT1, the 962, the 944? Sure you could say that the other cars like 787B and Sauber C9 make up for the GT1 and 962, since those aren't in Forza, but that doesn't mean the RUFs in GT4 make up for Porsche, at least not compared to how many Porsche's there are in Forza.
Anyway, here's to hoping for you guys that Porsche and Ferrari and etc. make it into GT5. Even though I really like GT4, I'll prolly only buy a X360 next-gen because of Project Gotham, a game that I've found to offer much more fun than the other more realistic games that compete in the racing-genre market.