The Monster Manual glossary entry on breath weapon says, "A creature is immune to its own breath weapon unless otherwise noted" (306).
What creatures are noted as not being immune to their own breath weapons?
I don't believe there are any! However: D&D 3.5 and the d20 system in general were written with the intention of being modular. The Monster Manual came with rules for modifying monsters and creating all new ones, so that people wouldn't find themselves limited by the options in the book if they suddenly wanted something else. So the reference to "unless otherwise noted" is there to give themselves, and the players using the Monster Manual, license to come up with a monster where this isn't the case.
For instance, I could create a kind of acid-spitting snake that was impervious to most standard means of attack as a way of encouraging players to try to get the snake to tangle itself in a knot and accidentally breathe on itself to destroy it. Okay, not the best monster, but thanks to the way that it's written I can use that as my starting point and improve it to something else.
Having said that, if I'm completely wrong and someone can find some other monsters that aren't immune to their own breath weapon, I'd love to see those.