Nope, I hadn’t thought to try another card, Fabrizio, but when doing so it worked! Import and playback were both normal. Thanks for the suggestion! So, with the problem being with the SD card, here are two new questions:
1) any suggestions for a replacement SD card brand and model? The card that failed was a Transcend 16GB SDHC, and the original that came with the camera was a Kingston 2GB SD.
2) I now have 4GB of AVCHD footage on the failed card I’d like to salvage. The files will open with VLC, and I just discovered they’ll also open with Handbrake, but not with MPEG Streamclip. Can you suggest a software tool for salvaging this footage in a way that keeps it looking great at economical file sizes, and also produces file types that are FCPX friendly?