i'd never do that
i work in a very relaxed, informal environment. my boss is the coolest possible. he'd probably discourage that kind of thing, but he'd never in a million years fire someone for that.
that would be idiotic. unless you deserve termination for other reasons, of course.
however, i still wouldn't say something like that, even though i don't consider it a big deal.
but it depends on the tone set by your boss, the client, etc...
you felt comfortable saying that? do others joke like that in email where you work (clients specifically)?