why aspect ratio is changing?

Watched our first blu-ray dvd last night on brand new pani bd-35 dvd player & brand new pani plasma 58" tv on thx mode. Great treat both in sound and vision. While watching The Dark Night the aspect ratio kept switing from letter box to full screen depending on the sceene. Connection was hdmi direct from dvd to tv. Can anyone help me understand why this happen. Thanks T. J.
Google "Dark Knight aspect problem" to see MANY responses...
Don't worry, that is intended. On the case it says:

1080p high definition 16x9 variable 2.4:1 and 1.78:1 (imax sequences).
Thanks i thought it was a hardware issue. I feel much better now! A vision from the director? ok I can dig it. I'll enjoy the movie more the second time knowing its not time to return equiment.My son and I were the only viewers that even noticed....and the only ones that know how to work the remotes!! Thanks again for the help. Happy Holidays T. J.
It had to do with its IMAX use.......you probably noticed the dialoge scenes were where it got small and then during big action stuff it opened back up........thats all IMAX issues......I amn no expert but I do know the IMAX movie went in and out of full blown video and I am guessing from there as to why its doing it on BRD.
Huh... thanks for the notes on the box. I wondered too but then remembered, it's a comic book come to life. In recent past if you look at the art work in comic books, er, graphic novels, sorry.... the art work is the focus. some frames are whole pages, others are thumbnails, some don't even get a frame about them.

It seemed a better depiction or feel of a graphic novel being painted onto the big screen... ala, Sin City or Hulk. sC used numerous shots in negative exposures, out of context frames, and odd angles just as they would have been on the pages of the novel. In Hulk, the close ups of only the eyes, panoramic scenes showing the Hulk and his Military Hulk wanna be foe in the midsts of war toern streets, decimated by the fall out of their battle were set to minimize their physical presence, and at the same time display the havok they had wrought, eg., in the final street fighting scenes between the two.

I think many of these out of the box artistic recreations which are being transfered from page to film, are being done exceptionally well and carrying the original intent of the graphic artists to a more accepted venu. Comics have long since moved on from orthodoxy... their films should reflect it. Many do. The Dark Knight went over the top with it's greater adherence to the pages it stemmed from.