Safari has a bookmark to a pretty extensive "iPhone User Guide". It's been there from the start, but I'm sure almost no user knows it. I don't get why they didn't just pin it on the springboard or something (well I do, it'd look kind-of ugly and you should be able to use the phone without a manual, still...)

I learned it the same as you from someone else. Now I use it so often, double-click to switch apps, that I can't remember life before it.