If I only had a dime every time I heard this "We will keep them separate" crap.
Here's the deal from the eyes of a 50-something electrical design engineer who has seen this crap go on for decades.
All the Android licensees are crapping their pants right now. Google is in full swing "cum-ba-ya, my lord" mode with these vendors because they realize they are critical to the long term success of Android.
The Motorola employees too are crapping their pants. Yeah, there are clearly two separate, but related, skill sets here, but there is also an enormous overlap. Google doesn't want these people jumping ship, so they will say anything and everything to keep them. Google wants to avoid a mass exodus of valued employees.
We will see promises on top of promises about Motorola Mobility being run as a separate company. We will even see it happen... for the first few years. Then... slowly... that hard line between the two will soften, then blur, then be incredibly difficult to find.
First, only senior management will report to Google. Then, the department heads will begin to report to their associated department heads in the other company. I say "other company", because it won't all be Mot reporting to Google. Merging companies successfully has been studied for years. What they will do is mix it up. More of a "blend" than an assimilation. There will be some Google departments finding themselves under Mot bosses.
The only companies that actually hold other companies in an independent way over the long term are holding companies... entities whose sole purpose is to hold companies, possibly repair them, and maybe sell them... or continue to hold them if it suits them.