I was thinking of buying a xD memory card adapter to plug into my expresscard slot (darn fuji and their proprietary selves).. it's about the only thing I'd use, anyways. there are countless adapters out there, though. firewire 800 is one of them.. I've also heard of some people using it to do eSata with external hard drives.
do a search for a couple of ports that you wish your MBP had, and I bet you'll be able to turn your expresscard into it!
I have a Belkin media adapter (SD, XD, etc.), and also a Delkin Compact Flash adapter. I plan on leaving the Belkin installed most of the time, as it fits flush in with the MBP case. I have a SD PC card doing the same in my PBG4's slot, so I'll have same funtionality when switching to the ExpressCard/34.
Got the Belkin at Fry's, about $35. The CF adapter purchased from B&H Photo, around $70.
Haven't been able to try them, yet, as I'm among the "patient" awaiting a 17" MBP C2D.