Certain journaled file-systems do suffer from performance penalties for hosting the journal on the same drive as the filesystem itself. Using an external journal can improve write speeds. With the advent of SSD's, it strikes me as entirely possible that a single SSD could support multiple external journals for filesystems hosted on traditional rotational magnetic media. While the Anandtech "SSD Anthology" did cover some of this, they didn't cover the "multiple journal" test-case.
Journals tend to be pretty small, so even a small, fast 32GB SSD could provide a lot of speed for multiple large file-systems. It would probably wear much faster than general I/O patterns out suggest, which is why having a larger device than you'll end up using is a good idea.
Have any of you done something like this, or even just used an external journal? I'm curious about real-world cases.