Apple had released the 10.5.7 software version last night but most of the users reports of problems with the update. And the entire download finishes, the update window reports that its some error with the software and its digital signature, and the update fails.

Here is the error seen when trying to update:

The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by “Apple”.

A few people say it installed when they downloaded the same update again, but a better way to do it is with the 10.5.7 Combo Update which is a 729 MB file that can help with your mac stability and helps with the update of Mac to 10.5.7 version.
I got scared for a while that was it the error with just my mac, but a small google search showed that i was not alone.
Download the combo update from here and install to update.