Helen Mirren Joins Glee
Helen Mirren (she doesn't count as a celeb guest star it seems huh Ryan?) is set to guest star on a January episode of Glee. The Queen will be lending her voice to a secret character. Details:

Oscar winner Helen Mirren has signed on to guest star in the show’s first episode back on Jan. 17, TVLine has learned exclusively.But in a twist, the British dame will be heard, not seen.
Per a Glee insider, Mirren provides the “inner voice” of a character who shall remain nameless. (For now at least.)
“Helen came in and secretly recorded several long and hilarious monologues two weeks ago,” says the source. “Then she visited the kids on set, and they had no idea she was coming; they were stunned.”
How’d the royal (and potentially recurring) casting coup come about?
The role was written for Mirren, but there was no guarantee she’d actually do it. So producers dispatched frequent Glee director and Mirren confidante Eric Stoltz (the two co-starred in the 2007 drama The Passion Of Ayn Rand) to sell her on the idea.
Stoltz apparently didn’t have to twist her arm too hard though. “Helen’s a big fan of the show,” says the source, “and the message it sends out about arts education.”