News of this particular maintenance update is significant for owners of PowerPC-based Macs, given that Mac OS X 10.6 Snow Leopard requires a Mac with an Intel processor. That means Mac OS X 10.5.8 or 10.5.9 will likely represent Apple's final push towards solidify and stabilize Leopard before shifting the majority of its resources towards the first point release of Snow Leopard, which will arrive this fall.
Rarely has Mac maker committed resources to providing further updates for its previous-generation operating systems after having launched a major new milestone release. Instead, successive updates are often limited to critical security updates and individual component compatibility fixes.
According to people familiar with the matter, roughly a dozen builds of Mac OS X 10.5.8 have been compiled to date, with the most recent builds reportedly standing as members of the 9Lxx build train. Mac OS X 10.5.7 development culminated with the release of build 9J61, suggesting that the 9Kxx build train was used for a customized milestone of 10.5.7 that bundles support for one or several of the new Mac notebook products introduced Monday.
At this time, it's unclear exactly which system components Mac OS X 10.5.8 will address. However, a cursory review of Apple's support forums shows quite a few threads from users who've been hit with network-related issues after upgrading to Mac OS X 10.5.7 (1, 2, 3, 4, 5).