With the recent Microsoft move consolidating efforts with Nokia (and there are rumors about HTC acquisition also), I think Google needed an hardware partner too, mainly to boost – even more – the Android development.
Since Android became not-so-open-source – decision that I understand and considering early developments was needed to give it stability and credibility – Google need to have control over the whole line of business.
Motorola by it’s side is, in my opinion, the manufacturer that have not-the-best Android UI (MotoBlur UI). I think Google will take over it and let the future MotoGoogle phones and tablets take the standard Android UI.
I think this partnership is a win-win for both Motorola and Google. But also if not already done, it may be pushing another huge vendor (read HTC) towards Microsoft.