That, and I think in English language movies and TV shows the subtitles are often transcribed that way.

It's probably songs, and it's not new. Consider "I'm Gonna Wash That Man Right Out Of My Hair," which IS the accurate spelling of the title. And if you search on YouTube and listen carefully, Mitzi Gaynor is not really saying "Gonna." I don't detect any trace of a "T" sound but she is definitely singing something like "go-inna." And YouTube has a 1919 recording of Nora Bayes singing "How Ya Gonna Keep 'em Down on the Farm (After They've Seen Paree?)" You could spell it "gonna" but it's much slower to a long "o" than a short "o," and there is a trace of an "i" in there. Helen Kane, 1928, "I Wanna Be Loved By You," and I admit what she sings is sometimes very close to "wanna" but I still hear some little catch like a very weak "t"... at 1:36, for example.

There was a discussion on this a while back: http://www.italki.com/discussion/81474 Yes, I also think it comes from learners watching media and listening to songs instead of communicating with actual native speakers. When I see people here write "gonna/wanna/gimme" (and let's not forget coz!), they're almost flagging themselves as non-native. Ironic, isn't it? :)

Add 'finna' to it, and it comes from talking with people.

I agree it's mostly the songs that contributed to it, followed by movies and maybe cartoons? And it reminds me of another similar thing in pop music. You see a lot of these "ticks" abbreviating words like 'round, o'er, hurtin', and so on. I know poetry traditionally had such things to help with its meter (o'er, 'round, Tis, etc). But the pop songs since the 50s and 60s brought about inordinate amount of abbreviating XXing to XXin', or even dropping the "g" altogether, both in transcription and pronouncing them. Bob Dylan songs are full of these for example, so much so seeing "hurting" instead of "hurtin'" is downright rare. I am just wondering whether it makes it look cool to most people, and if it's mainly Bob Dylan who introduced this trend or does it go much further back. And how do mainstream people feel about it? Do you find it interesting, or even enriching, or does it just annoy you? They didn't bother me much when I was younger, but as time goes on I seem to frown upon them more - I guess I'm getting more conservative :-)