Centrifugal is imaginary in a non-inertial reference frame (as the case is here), but is a "real" force otherwise.

There was a discussion on this exact subject recently on Reddit (/r/askscience) which might be interesting:

http://www.reddit.com/r/askscience/comments/m2b1t/how_fast_c...

and the entry on Wikipedia:

http://en.wikipedia.org/wiki/Centrifugal_force