Thanks to Dwayne Lamb of VisualByte in Toronto for digging up the definition of the term we have been using a fair bit on the TNX:TechNet Exchange Tour. The acronym AKU stands for Adaptation Kit Update, which is a set of code sent to OEMs to update Mobile 5 devices.
Like we said on the tour, you need Adaptation Kit Update (AKU) 2 to fully support all of the features of the Messaging and Security Feature Pack (MSFP) of Exchange Server 2003 SP2, including Direct Push Technology and remote and local wipe.
Thanks again to Dwayne for digging this up!!