What makes a unibody Mac better than what came before? I am looking for information about what macs are considered unibody and wondering if there are any notable advantages and/or disadvantages to this fabrication style. 

Answers that address marketing material as well as lifecycle data or experiences for largish deployments of macs with real life failure rates or data would be great to read.
The unibody design makes the case lighter and stronger. Since it is machined, there is minimal error in the creation of the case. For more information, watch the video on http://www.apple.com/macbookpro/design.html. The first unibody MacBook Pro was the 15" model released in 2008 and had a removable battery. I believe that all other unibody models (and only unibody models) have integrated, non-removable batteries.