Actually, I AM good, Grammarface!
“And what about you? How are you?” he asked me politely.
“I’m good,” I responded.
“You mean ‘Well.” Unless you’re a superhero or something, your grammar is incorrect.”
“HA! ACTUALLY, YOUR CORRECTION IS INCORRECT! IN THE IMMORTAL WORDS OF DARTH VADER, I HAVE YOU NOW! BEW BEW, KABOOM! HA HA!”

Cough, ahem.
Anyway, I was right, and I always said that I’d find some third-party source whenever people corrected me, but never really did because there’s quite a few more interesting things to do. But one floated across my screen, so I’m posting about it now, and I shall forever link to this blog post when people try to get me. Har har!
I mean, obviously this is more playful than anything because the number of people that actually care is quite small, but finally, at long last, my amusing vindication is here! And I didn’t have to go out of my way to look anything up! Huzzah!
3. It’s incorrect to answer the question “How are you?” with the statement “I’m good.” Wrong! Am is a linking verb and linking verbs should be modified by adjectives such as good. Because well can also act as an adjective, it’s also fine to answer “I’m well,” but some grammarians believe “I’m well” should be used to talk about your health and not your general disposition. (See episode 51 for more details.)
This is from the Grammar Girl, by the way.

March 9th, 2008 at 8:06 am
haha oh, jameson.
of course you’re correct.
you can say “I’m good,” just not “I’m *doing* good” unless you’re a do-gooder righting wrongs.
duh.
:)
March 9th, 2008 at 12:31 pm
Pah!
Assuming the question is “How are you doing?” and not “What are you doing?” then it would presumably refer to a status instead of an action in the same way your doctor might tell you that your health is good.
Duh back ‘atcha.
:P
March 12th, 2008 at 10:50 am
Your having way too much fun with this.
March 12th, 2008 at 1:58 pm
Heh heh. Maybe. ;)
March 12th, 2008 at 7:02 pm
Soo How are you? Heh. Well.. Just letting you know that “I’m good!” :P