Well, from my memories of discussions about the possible cache card, the idea was to keep the database cached for faster response times in the UI. The empeg doesn't really have this problem, and uses a very complex cacheing algorithm already which would be impossible to communicate to the IDE cache board. The real problem is the drive has no knowlege of what sectors you're going to want next...
All in all, if it's an open system, just add more memory and let your OS decide what to cache. If it's a closes system like a tivo, this is a Really Cool Hack.
Matthew
PS: I'm very happy to see that people are getting into the series 2 boxes. Makes my decision a bit harder on what tivo I need to end up with.