For balloon mappers: I found this note on SD card write speeds. Some of us had discussed the idea of using a slow SD card to limit writing speed and thus capture fewer redundant images when rubber-banding the camera trigger down.
There are two different specs: SD and SDHC cards. For all practical purposes the SD card are up to 2GB and have no speed classes as such but uses a speed rating such as 50X (X=150KB). There are currently four speed classes for the SDHC cards: 2, 4, 6 and 10. These numbers represent the minimum sustained write speed in megabytes per second. link
This would mean that with a high megapixel camera, say 3-4mb per image, it'd take probably 2+ seconds to write each image to the card using and SDHC Class 2 card. Given that most cameras in continuous mode shoot every second, that'd lower the # of images by at least 50%, no?