Ok, ok, so there was no DDOS. (I said there had to be something else going on, didn't I?) What actually happened is that I attempted an "svn import", which rapidly ate all my memory and started killing things. Today I tried again, and this time stuff other than apache died. Version 1.0 my ass.
Still unexplained is why there were 129 apache processes available to kill yesterday - on such a low-traffic box I've never seen more than about 10 running at one time.