I've seen some motherboards which in the description mentiona that a 22nm CPU "must" be used inorder to make use of 1600MHz RAM. Is this true for all motherboards, or is this for some motherboards only?
I'm not sure I entirely understand what you're asking.
But the speed rating on RAM is the maximum speed the RAM manufacture guarantees the RAM can be run at, it doesn't necessarily have to run that speed, it may run higher (with risk of system instability) or lower.
If you're talking about Sandy Bridge, the default speed is 1333mhz, it can be run higher if the mobo BIOS supports overclocking. Ivy Bridge, I believe is going to officially support higher frequencies than that.
So I guess, since I'm assuming what your question is, the motherboard may not support overclocking of the RAM if its not a certain CPU, however, this will not hinder your ability to install it and use it. The RAM will just run at the lower speed it supports.
For example I have 1600mhz rated RAM, by default my Phenom II runs it at 1333mhz, with my overclock, my RAM is running at 1480 something.