The SxS card driver from Sony is not compatible with the latest (and last?) Snow Leopard update 10.6.8, at least on some Mac Book Pro models. When inserting an SxS card into the Express card slot, the system crashes with a "kernel panic".
Apparently, Sony has no intention to fix the problem. This is what they say on their download page:
Attention (July 2011): Users of MacBook PRO* with Intel CoreDuo Processor (*Early 2006 Model: MA464/MA092)
Sony has found that a message will appear to say you need to restart your CoreDuo-based MacBook PRO when you update the OS to Mac OS X 10.6.8 (released on June 23, 2011) and insert an SxS Memory Card into its ExpressCard slot. We do not recommend that you update your OS to 10.6.8. If you have already updated your OS and could not switch back to the previous version (such as 10.6.7), please use your SxS card in an XDCAM or XDCAM EX video recording product, connecting it to your Mac via the USB cable. You may also be able to use your card with an 'SxS Memory Card Reader/Writer SBAC-US10,' which is sold separately.
If you have this problem, the only solution seems to be a re-install of the OS. Then, a system update to 10.6.7, and finally disabling all further automatic updates.
If you are thinking of avoiding a re-install and doing a downgrade from 10.6.8 to 10.6.7 instead, forget it. I tried, and failed miserably, ending up with an unbootable machine (kernel panic straight away at startup). But if you do know of another solution than a re-install, please leave a comment.