Ubuntu@Laptop: turning off internal speakers on connecting headphones

In most cases, Ubuntu has already configured everything.. just one check-box needs to be checked.

Open System->Preferences->Volume Control

There should be a tab named "Switches".
(If its not present, Goto Edit->Preferences and check "Headphone jack sense")

Goto the "Switches" Tab, and check the box in front of "Headphone Jack sense".
Thats it!!

If by default the options mentioned here are not there, then look for answers in the ubuntu forums.