Before we released our Direct Push solution no one really knew or cared too much what an Adaptation Kit Update (AKU) was. Historically an AKU was used to enable an OEM to deliver a certain device as the core Windows Mobile platform may have required certain additional functions to be added (for example landscape support in Smartphone for the i320 and Q)
Because we delivered Direct Push in AKU 2 - no everyone wants to know what's in each AKU :)
So the tricky part... the OEM decides which AKU they base their ROM build around. Therefore you cannot (as an Enterprise or end user) just update your device to the latest AKU without the OEM providing the ROM update.
MobileReview have published a detailed overview of the changes in all the Windows Mobile 5.0 AKU releases
You can read it HERE