Apple has released Battery Update 1.2 for MacBook and MacBook Pro models, and batteries for them, sold between February 2006 and April 2007. The update fixes some unspecified performance issues. Apple notes the issues that prompt the update do not present a safety risk (unlike the last year's major battery recall; see "Apple Recalling 1.8 Million Laptop Batteries," 2006-08-28). The problems appear to be a mixture of hardware and software, since another support page for the update notes a list of symptoms that could indicate a faulty battery (my favorite is "Battery pack is visibly deformed," a slight understatement compared to some recent photos of swollen MacBook batteries).
Be sure to run the update with the laptop's power adapter connected; when other batteries are inserted, they will be automatically updated. Apple also notes that Intel Core Duo-based MacBooks and MacBook Pros gain an additional two years of warranty protection for the battery. Battery Update 1.2 is a 244K download from Apple or via Software Update.