I think Spider-Man will take the opening weekend record again...they will open it in more than the 4100 theatres or whatever that DMC opened in and they will retake the record. The real trick is not how much Spider-Man will affect POTC3, but how much both Spider-Man and Shrek 3 will affect POTC3.
As much as I dislike Shrek, it does hold the highest grossing animated feature record and, for some reason, is adored by millions of people. I personally didn't like the first, didn't care for what I saw of the second, but do believe that the third installment will do better than most people, especially here, would like to think. It will do better than any of the Pixar/Disney or just Disney cartoon features that have been put out in the past 2 years...it'll even beat out the $260 million or whatever that the Incredibles made (note: did not look up info on the gross of the Incredibles - just going off the last number I think I heard)
The competition will be much stiffer for POTC3 next summer than it was this summer and that will definitely take it's toll on how much money it generates not only on it's opening weekend, but during it's entire run in the theatres. This can also be applied to Spider-Man and Shrek, but I see Shrek walking off with the summer....it's very kid-friendly (nothing scary, etc....except for the animation...blah), whereas Spidey and POTC have creatures and villians that will cause issues for smaller children.
Besides...kids love ogres that fart and burp......sigh