My "big" camera is a Nikon D50. It is a wonderful camera, but it is getting a bit dated and only takes regular SD cards, not SDHC. As I understand it, the SD specification is limited to 2GB, but the hardware can handle a full 4GB of addressing. The SD standards people artificially lowered the standard to 2GB either to move people to the SDHC standard faster or for some technical reason I don't yet understand. Are there technical issues with some devices and 4GB FAT16 partitions?
There is also evidently a requirement in the standard that SDHC cards be FAT32-formatted. Older cameras (like the D50) can't read this filesystem even if it were present on a small capacity card. The neat part of all of this is that some "4GB SDHC" cards are actually plain old SD cards electrically and just rebadged as SDHC since a 4GB card isn't allowed to have the 'SD' logo. What I'm saying here is all speculation on my part that I've pieced together from some very reliable web forums. :)
There have been some reports that, if you get one of these cards, simply formatting a 4GB FAT32 SDHC card as the older FAT16 will make it work in cameras like my D50. I really wonder if this new Anniversary Edition card is one of these special beasts. It would make sense for Eye-Fi to do this since "all" they'd have to do is replace the on board flash with a bigger chip and wouldn't have to change the card electrically to support true SDHC. For $79 shipped at Costco, I just may have to find out!