In an interview with Phone Scoop, Nokia Executive and Board Member Niklas Savander confirmed that Nokia's new deal with Microsoft gives Nokia nearly unlimited freedom to customize Microsoft's Windows Phone smartphone software.
http://www.phonescoop.com/news/item.php?n=7585

GSMArena backed this up with:

So in effect, the Nokia customizations to WP7 will be one of the following:
Surface changes that are quick and easy to accommodate in any new update by Microsoft
Deep-running customizations that will be integrated into the OS by Microsoft and made available to both Nokia and any other competing manufacturer

http://www.gsmarena.com/nokia_has_gr...-news-2340.php