No one likes waiting for software updates, but preparing such an update for release turns out to be a tougher ordeal than most people suspect. Sony Ericsson detailed their procedure of updating, which explains why we'll have to wait a while before they release the promised Android Ice Cream Sandwich (ICS) update, even though the ICS source code has been out for weeks. The first stage is to merge the changes brought about by ICS into Sony Ericsson's own source code and to resolve any issues that might arise from the merge.
GSM Arena, Sony Ericsson explains how hard it is to update to Android 4.0 ICS