Normally it is not a concern, if you set your exposure properly, but there are exceptions.

When you're using flash to stop motion, ideally you'd set exposure such, that if flash did not fire, you'd get a black frame - not enough light to expose anything. Then the only time you get real exposure - is when flash is fired. Since the flash itself fires at a fraction of a second on the order of 1/5000 - 1/10000 or so, that's going to be the acual exposure time. More than short enough to stop the hummingbird solid.

So on D90 and friends, just close aperture enough to not allow the ambient light kick in (can also lower ISO), and most likely you'd be fine.

That said - if you're doing this in relatively bright conditions - 1/500 sync speed will help you a little. As you probably know, required flash power is only determined by the aperture and ISO, not by shutter speed. So if you keep closing aperture/lowering ISO to get less ambient light in, you'd have to fire flash at higher power until you reach it's limit. Then you risk to underexpose. So in very bright situation - you may need either D40 with its fast sync speed, so that you can have an extra stop of free advantage, OR sb-900 with more power, or even both.

But the bottom line is - such condition is very unlikely, so most probably you'd be fine with D90 and SB-600, but yes D40 and SB-900 do have an edge in bright daylight.

LEM.