My previously mentioned problem—that the Sonnet Tempo-X eSATA 4+4 card's firmware is violently incompatible with Mac OS X 10.5—has been resolved. Owners of that card or other cards that use the same firmware need to be aware of the following: Version 2.1.2 of the firmware is 10.5 compatible. Apparently there was a problem with a 10.5 beta version, but that was corrected at Apple's end prior to 10.5's release. The reason that people, like myself, who went to the Sonnet downloads page to get the latest firmware for their cards received firmware that was not compatible with 10.5 was an error in the page markup. Specifically, if you clicked on the great, big down-arrow graphic to download the firmware, you received version 2.1.1 (which is not 10.5 compatible), but if you chose to click on the word "download" next to it, you received version 2.1.2 (which is 10.5 compatible). I've alerted Sonnet to the problem and it's probably been fixed by now. Hopefully, they've added a big, red note about this fact, since the version number of the firmware won't have changed to tip-off people about the difference.
One mild caveat: I had only a few minutes to try the Tempo 2.1.2 firmware with Mac OS X 10.5 before I had to leave for work earlier today, but they did seem to be working together just fine, which is consistent with what Sonnet technical support claims about the 2.1.2 firmware.
Follow-up: Yes, the 2.1.2 firmware works fine with Mac OS X 10.5.