Magic ward : Choose an opponent's zone. Treat the printed text box of each unit in that zone as though it were blank (except for traits) until the end of the phase.

If I play (and successfully resolve) Magic Ward at the beginning of battlefield phase on an opponent zone that has a developent that is a unit with ambush keyword, can he/she still activate the card for ambush?

My belief is that ambush was triggered by a development and the development do not assume the identity of a unit until the ambush step has been done, so ambush may still be triggered by the opposing player (without the benefit of Action it may have after ambushing, of course).

Would avoid using Magic Ward till all is clarified

if you play magic ward any time during the ballefield phase but before the ambush step the defender can't even trigger ambush because his units have no textbox

SleepyLynx is correct. The facedown developments aren't units, so their Ambush keyword isn't affected and can be used. Once they're faceup, they are units and affected by Magic Ward, so you can't use their Actions.

Thanks for the correction Mallumo

Thanks both Mallumo and Ellyrik! Though it doesnt prevent ambush, still loving its powerful effect