I personally wonder about this myself. I have seen references in "The
Devil's Dictionary" that claim it is Kerberos, but when I checked this
myself I only found the "Cerberus" variant.

I never actually heard of the "Kerberos" spelling/pronunciation until I got
involved with Kerberos myself.

From: Tom Yu <tlyu@MIT.EDU>

     "Cerberus" is the Latin spelling of the Greek "Kerberos", and
     according to the OED is pronounced like "serberus", but that is
     quite at odds with the Greek, as the initial consonant is a "k".
     MIT Project Athena chose to use the Greek spelling and
     pronunciation.

From: Jan Sacharuk <Jan.Sacharuk@cul.ca>

     Tom Yu is correct, Cerberus is the Latin spelling. However, the
     fact that the OED says that the 'c' is pronounced as an 's' is an
     English affectation. In Latin, the letter 'c' is always hard. So
     Cerberus is pronounced 'Ker-ber-ous'. The letter 'u' is also
     slightly different, making it somewhere in between 'oos' and
     'ous'.

From: Michael A. Covington <Michael@CovingtonInnovations.com>

     "Kerberos" is the original Greek name. In Latin, the letter K is
     not normally used, and in Roman times, C always represented the K
     sound. Also, "-os" is a Greek suffix (nominative masculine
     singular) whose nearest equivalent in Latin is the suffix "-us"
     (very familiar in Latin names). That's why the name goes into
     Latin as Cerberus.

     (See, a Ph.D. in linguistics is good for something! :)