ZDNet Ryan Naraine reports that 10.5.7 also incorporates Security Update 2009-002 which fixes 67 security vulnerabilities -- all copiously documented in Apple knowledgebase article HT3549. (KB article HT1222 outlines the security changes in Safari 3.2.3 and the Safari 4 Public Beta.)
The 10.5.7 update is available three ways, via Software Update (286MB), as a standalone download (442MB) and Combo updater (729MB).
I recommend installing Mac OS X updates from the Combo updater rather than the smaller "delta" updates whenever possible, as they seem to be more complete and generally cause less trouble than the deltas. You should also run Repair Disk Permissions (in Utilities > Disk Utilities) before applying the update and make a fully bootable backup before performing any OS update.
If you can afford to wait, are traveling and/or use your Mac in a production/business/demo environment you should wait a few days to a week before installing the update to see if any major issues arise. Use this time to verify that your mission critical software has been tested with 10.5.7 before updating.
NASA, for example, won't be installing 10.5.7 on its Macs until Atlantis is back on terra firma.
Apple makes the following recommendations:
John Nutzman reported getting an unusual error dialog saying that the 10.5.7 update "can't be saved" due to an incorrect digital signature. Weird.
iClarified notes that Apple has fixed DFU Mode and enabled iPhone notes syncing in the just released Mac OS X 10.5.7.
How's it working for you?