In a bit of an about-turn, Microsoft will allow Nokia to "customize everything" in Windows Phone 7 in order to both differentiate it from handsets made by current WP7 OEMs, such as HTC and Samsung, and presumably to ensure smooth integration with Nokia's existing services.
Microsoft had, until now, kept a draconian grip on the look and feel of Windows Phone 7. In fact, other than being allowed to customize a couple of tiles on the home screen, OEMs aren't allowed to change anything on WP7! This change not only means that we'll see distinctively unique Nokia-Microsoft phones, but also that Nokia will retain more of its identity than just an embossed logo on its handsets.
The big question, though, is whether this represents an exclusive deal for Nokia, or a complete shift in Microsoft's WP7 strategy. You can be sure, however, that HTC, who was forbidden from installing its Sense UI on its WP7 phones, will be quick to question Microsoft's new stance.