I think quite a furore has errupted!!!
I would like to say that if an ob does your ultrasound and says it is a particular sex and feels quite sure about that it is more likely that you will get what you are told you are having...

Once you get further on past the 28 week mark it can be more difficult to tell the sex on ultrasound as there is less room and your baby is more likely to not show her bits just due to the space issue.

I am not sure about the 3D ultrasounds. It makes sense to me that they would more easily show gender but that is only an opinion and I don't really know...
I guess a good rule of thumb is to say okay it's looking like XY but it could be XX and be open to either...