The performance issue was discovered after users of these particular models upgraded to Mac OS X 10.5.8. From the release notes:
This update is recommended for some Intel-based Macintosh computers running Mac OS X 10.5.8 and addresses an issue with AirPort performance while running on battery power. It may only be installed on:ResourcesMacBook (13-inch, Late 2007) MacBook (13-inch, Early 2008) MacBook (13-inch, Late 2008)
MacBook Pro (15-inch, Early 2008)
MacBook Pro (17-inch, Early 2008) MacBook Pro (17-inch, Late 2008)
Read this lengthy Apple Support Discussions thread regarding the poor AirPort performance of certain models of MacBooks and MacBook Pros after the Mac OS X 10.5.8 update.
UPDATE (8/14/09): Apple acknowledges wireless problems with knowledgebase update
In the days following the 10.5.8 update, it was quite apparent that the wireless issues affecting people were due to a problem with the update and not a result of third-party applications or customized system configurations. Various approaches to fixing the problem clearly showed it to be with the "IO80211Family.kext" extension, and Apple updated this with the release of the AirPort Client Update 1.0 patch.
Today Apple released an official knowledgebase document on the problem, which briefly outlines the computers affected by the bug and mentioning the update as the method for fixing the issue. There is no new information in the article than what we have mentioned here at MacFixIt, but it is great to see that Apple addressed this problem so promptly and thoroughly by acknowledging the problem and getting a patch out within a week of its discovery.