I was up at the graveyard, getting Sunny's weeds. After clearing out the area, I was "exploring" (actually trying to find the last dead body). I must have gone too far past the "Do Not Enter" signs at the top of that hill.

Suddenly, my game stops and I get this message: "Do you want to change anything about your character before going on?"

So, I'm wondering, is Sunnyvale a big Tutorial?

You mean Goodsprings. Kind of, it's the starter area from the cemetery down to Goodsprings source out to the road area. It's a change to change appearance, SPECIAL, skills and traits. Those signs are there for another reason by the way...

Yes. Goodsprings! Still stuck in FO3, I guess

No, Sunnyvale is mentioned in New Vegas. Not sure where you're thinking of in Fallout 3!