No catch here. I ordered mine last night for $0 with $0 shipping.
PalmOne is giving these out to make up for the snafu with NVFS, namely that it uses inefficient 512 byte clusters, which waste a ton of space for small files. Note that this is only for GSM Treos (and will likely go away when PalmOne releases a new firmware to address the issue). Sprint Treos have already had a new firmware that addresses the issue and thus they are not eligible for the free SD card.