I don't know about filesystems. But as was mentioned earlier in this thread, Linux does now have bcache -- a generic block layer caching system that can be used with most filesystems to cache stuff on an SSD in front of the main (eg. mechanical or network) storage media.

I think some of the add-on filesystems can do this kind of thing as well -- not mhddfs, but the similar commercial one (forgot the name..) discussed previously can use an SSD as a front end cache as well.

EDIT: unRAID is probably what I was thinking of.