After updating to OSX 10.8.2 (Server), internet sharing stopped working. After a lot of fighting, that included setting up manually (turned out not to be viable because I couldn’t figure out how to automatically launch an ad-hoc network at startup), I undid everything I tried except for this one tip:
http://support.apple.com/kb/TS4418

Perhaps if I’d done that, and only that, I would have solved it quicker. But it’s hard to know, and hence quite frustrating (nevermind disappointing, for an OS where everything “just works”).