Aside from photo transfers and straight up storage expansion, the SD card slot in the new MacBook Pros has a single, extremely cool trick up its sleeve slot: it's bootable.
According to a fresh Knowledge Base article, a properly formatted SD card can be used to boot OS X from the internal slot. As (vaguely) described, this could be a valuable backup and restoration tool, or a convenient way to maintain a semi-portable install of your OS. In proper giveth/taketh fashion, Apple states in the same posting that, despite being the clear, inevitable replacement for the SDHC standard, SDXC card's won't be supported, meaning storage will effectively be capped at 32GB.
If Apple really wanted to make this useful, they'd extend Boot Camp to support external storage devices; a feat which is currently only manageable through complex and unwieldy hacks. On this, the article is tellingly silent. [Ars—SDXC tip from Richard L.]