It probably isn't a big surprise to many, but you can officially boot the new MacBook Pros from an SD card according to this KnowledgeBase Article (via Ars).  On older machines, you could boot from a USB external drive and it looks like that SD card dock is connected through the USB connector, so there aren't too many tricks at play here. 

To create a boot disk, you'll need to format the card to Mac OS Extended using Disk Utility.app which will make it mostly useless with PC's and cameras.  It will also be many times slower than a standard hard drive or SSD for boot-ups.  However, this could be a great tool for diagnosing issues with hard drives and running diskutils.  Diskwarrier should come on SD...in a few years.

Interestingly, you could also use those little SD cards as backup drives.  If you just want a quick and easy Time Machine/automated CCC of your important docs, it is hard to imagine an easier solution.  With a 32GB card going for $70, or a 16GB card coming in around $30, it is a small investment to keep your files safe.

Oh, and those with older MacBook Pros with ExpressCard slots?  You can be part of the fun as well with an ExpressCard to SD card reader ($28) - which turns your ExpressCard slot into an SD Card reader.  Or eliminate the middleman and just get an ExpressCard SSD.