Apple has accelerated considerably the rate the beta versions of Mac OS X 10.5.7. They now follow one another every 3 or 4 days, each time fixing more problems. One seems very close to the final version and Apple wants obviously to release it as quickly as possible. This eagerness may be related to the arrival of the first iMac equipped with Radeon HD 4750 that would need the ATI drivers.
If Apple does not shift from its normal behaviour, it could be released Monday evening with certainly of a following security update for Mac OS X 10.4.