Officially announced in a commercial during the Superbowl - Prince will be appearing at next week's Grammy's. He's not the 'surprise opening', but will definitely appear.

[japartington adds:]
but sources said a big show-opener is planned, courtesy of Prince.
The rock/pop/funk/R&B legend is nominated for best pop instrumental album for "N.E.W.S.," which didn't make many headlines at all. But it will be nice nonetheless to see His Royal Purpleness once again on network TV.
There are two other live performances of note scheduled by multiple nominees. Detroit's blues-rock duo the White Stripes are set to play, and the Atlanta hip-hop duo OutKast will perform along with members of two of the greatest bands in funk and R&B history, George Clinton's Parliament-Funkadelic and former Chicagoan Maurice White's Earth, Wind & Fire. -Jim DeRogatis