While I agree with Faldage that "I seen" is not exactly an uncommon usage, I do not agree with his take on the development of that usage. "I seen" has been around in America for a long time - I remember reading it in quotes written in the 19thC. "I seen" and "I been" seem to have more origins in African-American and poor Southern speech patterns than anything else I can think of. At least, that's where you see them used the most. You very, very rarely hear it, even from kids, in Zild. It is, however, part of the verbal shorthand used by some brogues here in England.
I don't think that anyone believes that it is a "valid" grammatical usage, however. Like everything else in English, it seems to be negotiable. The interesting thing is, always, that even when we make the most diabolical blunders in the use of the language, we seem to be able to understand each other on the surface. Someone saying "I seen that movie" may make me grit my teeth, but I understand what it is that the ignorant sod is saying.
And when it comes from socio-economic or ethnical/cultural groups where it is an accepted redefinition of English, it doesn't even seem that offensive.
None of this applies to you lot, my friends!
