I think it'll make a decent amount of $$$ - because with such a wide fan base, the flick's 'got legs'. The only other 'big' movie out for the rest of the summer is that 'Bourne Ultimatum' for competition.
Ordinarily, I would contend that the 'law of diminishing returns' would apply for the Simpons movie. That is, (particularly with so-so or bad word of mouth or poor reviews) if a summer movie yields around 50 million opening weekend, it will drop 50% the next weekend and so on. Usually this means it will top off at a little over 100 million ('Ghost Rider' and 'Fantastic Four' are excellent examples).
However, because August is such a lean month, and with the antipication - I think the Simpsons with open at around 60 million, and top off at around 150 million.
I'm thinking about 60-70 million, considering that there are a lot of people on the East Coast who go to the beaches these mid summer weekends rather than the theater. If this movie came out in late May, it would have done 90 million easily.