In an interesting show of "openness", both Sony Ericsson and Motorola yesterday published blog posts detailing the process they go through to bring their existing devices up to new versions of the Android operating system. With both blog posts published on the same day, it's hard to believe that there wasn't some sort of coordination between the two manufacturers, although Sony Ericsson's post was much more detailed than Motorola's.
For each major new version of Android, Google works with one manufacturer to create the flagship device, called the Google Experience Device. To get the Google Experience Devices out, usually just before the source code is publicly released, the chosen manufacturer will be involved in the development cycle of the new version. Popular belief had been that other major manufacturers (think the likes of Samsung, HTC, Sony Ericsson, LG, and especially Motorola - now very close to being owned by Google) also got some kind of early access to the source code, especially as these are the companies who build Google certified devices with the full suite of Google apps onboard. However, both company's blog posts confirmed (Sony Ericsson's more explicitly than Motorola's) that they too only get access to the new source code on the day it is publicly released. This puts them on the same level playing field as any other lesser known manufacturer who fancies building devices from the AOSP (Android Open Source Project) code only, which is where most of the cheap and nasty tablets have risen from in the past.
Sources: Sony Ericsson Blog, Motorola Blog