I too have only ever used 'forgotten', but as the poster had clearly picked up on the posibility of both versions, I thought it wise to check my trusted (2002 edition) Oxford Dictionary. This is the Oxford's exact definition (without examples). The underline is mine:
'forget: v. (forgets, forgetting, forgot; past part. forgotten
Forgotten is more frequently used in the U.S. both verbally and in writing, and forgot is rarely used. The same is true with gotten/got. E.g.: I have forgotten all about it.