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.