Let me preface what I am about to say. I am a huge Disney Theme Parks buff, and snort my share of pixie dust yearly. However, I was able to ride Harry Potter and the Forbidden Journey about two weeks ago for the very first time. In my opinion Disney has created nothing on par with this attraction starting with the queue and ending with the ride. I walked out of this ride and thought "this is what an attraction in the year 2010 should be like". It was absolutely amazing!
If US continues to add on to their park over the years, and it is similar to the quality of Harry Potter Land, then I think attendance may begin to shift in Uni's favor. Not anytime soon, but years into the future. When I was little my parents took me to Disney because it once amazed them, and they wanted to share it with me. I now go to Disney because it is nostalgic, and it also amazed me as a child. I will likely repeat the cycle of bringing my children to Disney (when my DW and I have them) the way my parents brought me, but I will also take them too US.
If I were a child now I would be much more amazed with HP than anything that is currently at Disney. As a result, there is a good chance that if I were born today, and US continued to expand using quality similar to Harry Potter, I would grow up nostalgic for US more so than Disney. I would then take my children to US, and they would repeat the cycle by taking their children.
In short, I hope Disney begins to innovate and raise the bar the way they used too. As it stands now Disney is still the better vacation destination, but if they stand idle, many years down the road, US will have a chance at taking a big bite of out of Disney's Theme Park market share.