If I found out that the person was hiding it from me and had no plans of telling me until after marriage, THAT would be grounds for a resounding no. If I found out before dating him, I would be concerned (maybe a little more so than in the case of divorce), but if there were no other obvious issues at play, I think I would go out with him, because you really have to meet a person to see how they personally deal their depression. For some it’s just something that happens that they can deal with, but for some it’s indicative of much bigger issues that can’t be ignored.
I think, as SaysMe says, the most sensible course of action is not to tell them right off (because of all the resounding NOs), but to tell them as it begins to get serious (not too late, either), so the person has a chance to make first impression based on their perception and make a fully informed choice.