I welcome the 4GB drives as well, but there are a couple of things to keep in mind.
Firsly, these will not work in all cameras. The camera's OS needs to know how to read and write to a FAT 32 file directory. At the moment only the Canon 1Ds and, I believe, the Kodak DCS bodies and backs know how to do this. Future cameras likely will all have this capability.
The other concern is putting so many eggs (or images) in one small basket. Between 400 and a thousand frames on one card is an awful lot to put at risk. I think I'd rather have 4 1GB cards than 1 4GB card, just from this perspective.
Also, 1GB Microdrives have become incredably inexpensive. Under $250 many places.