I've been involved in an $1B+ acquisition that stretched out over 12 months. The initial deal, that was announced publicly, had language of the form "All assest required to run the subsidiary will be transferred to the acquiring company." The next 3-6 months required us to determine exactly what was actually purchased. And finally, the last 6 months was us transferring those assets.
The CEO doesn't get involved in the line item details - just announces the high level strategic decisions.
There is a likely some perceived value, to both Microsoft and Nokia, in communicating this strategy to the market/developers/partners - I'm not sure _what_ that value is, mind you, but that would be one reason why the high level deal was announced before all the nitty gritty details were inked.