Breaking $400m on it's 18th day will be tight; depends on what the weekend *actuals* end up being.
Regardless, whether it breaks $400m on day 18 or 19, The Dark Knight will have broken the old record to $400m by almost 3.5 weeks.
These numbers are really unprecedented.  SM3, despite having a similarly large opening weekend, didn't even come close in regards to the daily numbers.  Most mega-blockbusters like SM and POTC sprinted up to the $200m mark, galloped to $300m, and jogged to $400m.  The Dark Knight is still sprinting.
The biggest opener this week is Pineapple Express, a stoner-comedy.  Assuming another 40% drop for The Dark Knight, that puts it around $26.5m this coming weekend, giving it at least a lock on #2 and a good chance at #1 for another weekend.