In addition to the previously reported Windows Vista SP1 drop to testers, Microsoft has just recently (few days ago) dropped a copy of Windows XP Service Pack 3 in the lap of the same small group of testers.
Coming in at less than 350MB, the service pack includes fixes for over 900 reported problems, some of which have already been resolved with post-Service Pack 2 hotfixes.
For the hardcore build string collectors, this build is tagged 5.1.2600.3180 (xpsp.070718-2058) (Thanks again C).
Given the close proximity of the two betas we guestimate both 'packs will be released simultaneously. We'll let you know if we hear of a more solid release date.
Source
Screenshots
Vista SP1 and XP SP3 Released To Testers (screenshots included)
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