In my neck of the woods, Wifey refers to a significant other (usually of the female persuasion) that is a very serious girlfriend, but who is not yet a fiancee or a wife.

It is a term that carries no negative connotations, but may be said to imply that the girlfriend is of such a serious nature that you might as well be married. In closed male-only circles, this usually corresponds to "the ol' ball and chain." At least, it should stay in closed male-only circles; the few times it has reached female ears has resulted in disaster. The ball and chain thing I mean, not wifey.

The Urban Dictionary agrees with this definition (sic);
Wifey - a girl who you dont consider strictly as your girlfriend, but someone who u got a possibility of a future with.

Usage: Yo, thats my wifey, not just another girl.
"Wifey" is also the name of a novel by Judy Blume. ISBN: 0671693816, it is a sexually explicit romance novel of 288 pages and comes in a Soft Cover or Hard Cover format. I haven't actually read it myself, but the 67 reviews found at Amazon.com say it has an average of 4/5 stars.

"Wifey" is also the name of a song by the artists Next. Here is a brief excerpt:
You're never trying to eat
Always stood right by me
Make living lively
Highly spoken of
My only love
The only one, you're my wifey
Make me life complete, sweet
But you know when to flip it street
Freak but only when it comes to me
See that's why you're my wifey
I've never actually heard the song myself, and I don't really understand them crazy lyrics either. It's just some guy lovin' his wifey.
Sources: urbandictionary.com, musicsonglyrics.com, amazon.com, my life experience