Before we left on our trip, I picked up a pair of 4GB SD cards for my point-and-shoot Canon Elph. I brought my Linux netbook along, which has a built-in SD card reader, and the cards worked fine with that.
But they didn’t work at all in any of the external card readers that I have, and I had to resort to a USB cable to transfer photos from the camera to my other computers.
Wikipedia discusses compatibility issues with 2GB and larger SD cards, which is initially what I thought the problem was. On looking more closely at the cards, I see that they’re the newer SDHC cards. The card readers were all a few years old and must have preceded the SDHC standard.
I spent $7.99 on a new card reader this evening, which supports SDHC and most of the other small card formats.