I don't know about that. In the game that I was playing, I was able to conquer 15+ cities without it affecting my happiness except by a few points, which I quickly recovered, because I already had the right social policies in place, and a lot of excess happiness to take some damage.
Its already late in my game, but its more due to my style of playing than having large number of cities. I think it has to do with doing things at the right time, and its probably possible to do what I did earlier in the game.
I don't really agree with the happiness mechanic though. Neither did I agree with the corruption mechanic in Civ4 which was put in to restrict you. The way Civ designers have tried to restrict the amount of cities you have has always been too artificial imo
Civ4 did not have the corruption mechanism that was Civ3. And I completely agree that Civ3's corruption mechanic was bad. Cities that were far away were totally corrupted away to uselessness until the advent of new government types.
Civ4's mechanic was maintenance cost per city which is higher the furthest away you were from the capital. Therefore it affected your cash flow on a national level but not on an individual city level. I found Civ4's mechanic to be very sensible and a good change IHMO as it punishes expansion if done too quickly before you have enough infrastructure to support it.