I use "G'day"...well, it's near all that I use as greeting. I use it on posts, PMs, emails as well as spoken. It is no affectation, but automatic, reflexive, just a natural part of my vocabulary and conversation.
As for the banner, well, a constant presence in Australian culture is a broad sense of irony, one that would appreciate the humor in intentionally advertising directly to our cringe factor. My first thought on seeing it was the same as I thought when watching the episode of The Simpsons, Bart Vs. Australia - that it was consciously cliched and heavy-handed, like a nudge and a wink to "true blue Aussies"
about international perceptions which we have sometimes ourselves been complicit in generating. It was a little "shrimp on the barby" Google was slyly grinning at while presenting to us.
IOW, I thought it was funny.
Cheers, and g'day,
Marc