Well these days, studios tend not to schedule their major release to open on the same day as another studios major release.
That's a result of the high concept blockbuster economy, and why Hollywood studios will never release films like they did in the early 1980s. With studios relying on fewer and fewer films that are more and more expensive they need to be able to dominate the weekend (and, if possible, the following weekend).
Releases of big films like these are saturated and heavily advertised -- which means the vast majority of their box office, unless they become a major hit, is earned in the first week. Studios can't afford to split the box office with another movie, which is why you see films like The Bourne Legacy being delayed a week. Universal doesn't want to lose out on potential box office receipts because of Warner Bros.' The Dark Knight.
I suspect The Dark Knight Rises will clean up the box office for at least a fortnight, if not 3 weeks. It could be argued that it is this years Biggest release.