I was really frustrated this morning. I've seen the new Gmail interface, I like it, I love some of the new shortcuts ([ and ] rock my world)... but it still wasn't enabled on my gmail account.
Just on the off-chance it was a problem, I checked my language setting. Yep, I had English (UK) selected - as soon as I switch to English (US), I've got the new interface version.
If you're also frustrated, you might want to try twiddling your language as well.