Major partners in Symbian have agreed to not allow Nokia to own over 50% of Symbian. A deal back in February to buy out Psion's Symbian shares would have left Nokia with 63.3% of Symbian.
Siemens, Sony Ericsson and Panasonic all upped their investments to even out the ownership. Here's the current rankings (after the Psion deal and today's move). Prior ownership numbers follow in parentheses.