I'm not normally all that pedantic, and am usually quite a fan of the Oxford English Dictionary but this has kind of bothered me a bit, would be nice if they were a little more equal...
Definition of Housewife: a married woman whose main occupation is caring for her family, managing household affairs, and doing housework
Example: the traditional division of labour between the husband as breadwinner and wife as housewife. I am not just a housewife, I am an accountant, nurse, negotiator, cook, driver.
Definition of Househusband: a man who lives with a partner and carries out household duties traditionally done by a housewife rather than going out to work.
Interestingly, SAHM is only included in the US Dictionary (as Stay-At-Home-Mom), but there's no entry for SAHP or SAHD.