There were some threads about the Eye-fi on Scubaboard. If I recall the gist, it was fairly slow in transferring, could only transfer to one laptop (e.g. it didn't really just link in wireless mode to a wireless 'network' like a laptop itself does, but had to link to a specific device), and there were obvious battery issues (wirelessly transmitting must be done with the camera on supplying power to the card). I was one of those somewhat enamored of the idea of not having to crack the housing to yank the images, but once I thought thru the speed issue it became a non-starter solution.
As far as SDHC, I'd think if the camera is SDHC compatible it will be regardless of the *size* of the SDHC card, since the SDHC is a standard. 4GB, 8GB doesn't matter, it'll be FAT-32 formatted (or whatever). Just don't try buying a NON -HC type SD card in above the 1GB type size if the camera wants to see SDHC for larger cards. At least that's how I understand it. Internally I don't think there's a lot of difference between the actual silicon that's in the SD or the CF formats, the CF is just a bigger slot and interface still. (You can often find converter adaptors for SD cards that let them fit into CF slots, which I think proves this point out).