Back in the very first episode (chrono order) when Kyon gets his new window seat and "sings" to himself "goodbye Haruhi,
foreverrr", I've felt all along that it's a reference to this scene from the (old!) Marx Brothers movie A Day at the Races:
http://www.youtube.com/watch?v=Y_prmS_q2oU
(the whole scene is funny, but I'm referring to 1:50)
That's always been a favorite comedy of mine, but I just discovered that it's a line of "real" music, not improv'ed:
http://www.youtube.com/watch?v=-QSa8vi1Yn4
(3:30)
Kyon's "forever" isn't exactly the same, but it seems close enough to me.
Edit:
Wow! I just looked up the lyrics:
http://www.recmusic.org/lieder/get_t...l?TextId=37781
Other lines include:
Goodbye Summer! Goodbye! Goodbye!
All the tomorrows shall be as today
eerie, huh?