If you had installed the Safari 4 Beta, the update to 10.5.7 does not “reset” you back to Safari 3.x. Safari 4 Beta itself has also been updated. The update appears in Software Update after you have upgraded to 10.5.7.
More information is available at MacNN, as well as, of course, Apple’s release notes.
One of the biggest improvement I am hoping to see with Mac OS X 10.5.7 is more reliable Bluetooth Modem connection via my N95 8GB 3G data connection. Right now, every now and then, Mac OS X will suddenly decide not to work with the modem connection, and I have to remove the Bluetooth pairing and recreate it. It’s probably faster than a reboot (which wouldn’t fix it anyway), but it is still a hassle.