For guests buying one-day tickets I could see Potter affecting the choice between DHS and Universal.
But the larger impact isn't so clear. Potter's opening could mean more Orlando visitors in general, with many of the additional visitors spending a day or more at WDW parks.
And it's no sure thing that if AK's attendance did drop significantly for any reason that Disney would respond by sinking a lot more money into the park.
The way my family thinks about it is, "When Potter opens, how can we get
over there for a day during our next Disney vacation".
Not "we need to travel down to Florida to go to Harry Potter".
I imagine there are Potter fans that will go to Florida, maybe for the
first time--but you can't tell me they won't be thinking Disney World
in their trip plans.
So I'm thinking, a trip for Potter, is a trip for Disney, anyway.
I'd think that Disney is thinking the same thing.
This is a common mentality. Universal is a diversion on a Disney vacation. I'm sure there are people that visit central Florida just for Universal, but the majority of people will have Disney be the focal point of their vacation. The attendance numbers support this.
When Islands of Adventure opened, it brought more people to Central Florida, those people all went to Disney, and some went to Islands of Adventure
instead of Universal Studios. Universal saw the majority of Islands of Adventure's guests come from it's main park. It hit Universal Studios hard, and really didn't impact Disney at all.
This is why I don't see Disney panicking about Harry Potter. They are continuing with their slow and steady development, knowing full well that if they need to react they will. I don't think the Fantasyland expansion is really intended as a way to combat Harry Potter, I think of it as business as usual. If a $300 million investment into Harry Potter does wind up making a dent into Disney's central Florida attendance numbers than I fully expect their reaction to be even more substantial than the Fantasyland expansion.
Here is the current timeline.
2010: Wizarding World of Harry Potter opens to the public
2011: Star Tours 2.0 opens to the public
2012: Fantasyland Expansion opens to the public
2013: Pixie Hollow expansion opens to the public
That timeline is based on some projections, from each company - but by no means do I think Disney will sit solely on the Fantasyland expansion if Harry Potter hurts their attendance. If that's the case, then I think we see a few more things happening in the 2012-2014 range.