In case you were not previously aware, the word "ain't" is most definitely a part of our English language, though how long it has been is unknown to me because I only recently found out that this word that we were forbidden to utter in grammar school can now be used freely by anyone who chooses to use it.

From The Merriam-Webster Online Dictionary:

Main Entry: ain't
Pronunciation: 'Ant
Etymology: contraction of are not
Date: 1778

1 : am not : are not : is not
2 : have not : has not
3 : do not : does not : did not -- used in some varieties of Black English

Usage: Although widely disapproved as nonstandard and more common in the habitual speech of the less educated, ain't in senses 1 and 2 is flourishing in American English. It is used in both speech and writing to catch attention and to gain emphasis.

"...the wackiness of movies, once so deliciously amusing, ain't funny anymore..." -- Richard Schickel
"I am telling you--there ain't going to be any blackmail." -- R. M. Nixon

It is used especially in journalistic prose as part of a consistently informal style.

"...the creative process ain't easy..." -- Mike Royko

This informal ain't is commonly distinguished from habitual ain't by its frequent occurrence in fixed constructions and phrases.

"Well... class it ain't." -- Cleveland Amory
"...for money? say it ain't so, Jimmy!" -- Andy Rooney
"You ain't seen nothing yet."
"That ain't hay."
"Two out of three ain't bad."
"If it ain't broke, don't fix it!"

In fiction ain't is used for purposes of characterization; in familiar correspondence it tends to be the mark of a warm personal friendship. It is also used for metrical reasons in popular songs.

"Ain't She Sweet"
"It Ain't Necessarily So"

Our evidence shows British use to be much the same as American.