Secure Digital cards with capacities of 4GB and greater are coming. Given that most SD cards seem to have topped out at the 2GB mark, this could be a good thing. Known as SDHC (Secure Digital 2.0 Hi Capacity), the cards should reach capacities as high as 32GB. Unfortunately (you knew that there had to be a catch), they will only work in devices that come with a SDHC-compliant slot.
Panasonic and PQI are set to release 4GB SDHC cards which will support transfer rates up to 6MB/s. Prices are expected in the vicinity of $250 USD.