Assuming you are referring to read cache, making a ram disk is pretty much pointless because the memory is already being used as such (ARC). You would be over complicating things for nothing. Someone can probably elaborate on this but I would recommend doing a few google searches, there are some great articles that explain how each different form of cache works in ZFS.
As spankit has said, ZFS manages RAM extremely well with regard to using it as an adative read cache (ARC); adding a RAMdisk is adding an unnecessary layer. Using a RAMdisk as L2ARC will also reduce the amount of ARC available as L2ARC eats up a small amount of RAM which would otherwise be used as cache... so you'll end up with more complexity and less RAMA available for caching.