If corsair are happy to RMA the RAM do it, but make sure you are in agreement with them that if they test the RAM and it is not at fault that you are not out of pocket, your statement seems to relate to this..
The thing about RMAing the mobo is that you haven't really tested is, coupled with the fact that ASUS RMA's are one of the worst to do, it takes a long time and you make have to fork out a fair bit of money if its tested to be working correctly.. This statement is from hear-say only not personal experience.
So I think you have 3 options,
1. Buy / Borrow some DDR2 to test if the controller is broken.
2. RMA the RAM with Corsair, you have tested and the technicians have already looked at your case on their forums.
3. RMA the mobo with Asus with no real testing of your own.
Unless you try the board with new RAM its not worth chasing ASUS for an RMA.
Good luck, stories like this help boost respect for the company's involved with their support/return system.