Well given that he utters it post regeneration, and is seen to say it again in the S5 trailer then we can maybe assume that he'll say
Geronimo...A lot.
Fans of Moffat's work, and in particular Coupling might recall that Steve tells Susan early on in their relationship that he had an ex who had a really hunky ex-boyfriend who used to shout Geronimo upon...well when attempting re-entry shall we say
Anyway, I likes it, a damn sight cooler than "I'm sorry, so sorry" that's for sure!
Geronimo...A lot.
Fans of Moffat's work, and in particular Coupling might recall that Steve tells Susan early on in their relationship that he had an ex who had a really hunky ex-boyfriend who used to shout Geronimo upon...well when attempting re-entry shall we say

Anyway, I likes it, a damn sight cooler than "I'm sorry, so sorry" that's for sure!