Apple has released firmware updates for a number of Macintosh models, including the (2.3 MB),  (1.8 MB),  (3.1 MB), and  (1.6 MB). In each case, all Apple will admit to is that the update "fixes several issues to improve the stability" of each Mac. Otherwise, I've seen one report that the update fixes problems with the MacBook Pro's closed lid mode. Late in the week, Apple also released the  (1.2 MB).
It doesn't appear as though these firmware updates apply to all models of the MacBook, MacBook Pro, and iMac; the updates don't appear in Software Update and refuse to install manually on at least my MacBook and Tonya's MacBook Pro - both early versions of those models. Apple says nothing online about which models need these updates, so I recommend relying on Software Update; if a firmware update appears in Software Update, install it, but if not, don't bother trying a manual installation.
To complete the firmware update process, follow the instructions in the updater application that launches automatically after the installer finishes. And, though this should be obvious, don't do anything that could interrupt the process. If you do, you'll need a Firmware Restoration CD to restore your Mac's firmware. Apple just released (30 MB), which supports the Xserve (Early 2008), MacBook (13-inch Late 2007), iMac (20-inch Mid 2007), and the iMac (24-inch Mid 2007). Previous versions of Firmware Restoration CD support other models, although it's not clear if all models are supported.