I use Quick Settings and there's an option to toggle auto-rotate. I have it so the shortcut to Quick Settings lives in my Notification bar so to toggle it requires a swipe, two taps and the back button to toggle it and get back to the app I was in.
If you wanted to reduce this down to
tap home, toggle, tap return if you use QuickDesk which lets you put widgets on it's popup launcher.
I'm sure you could probably set it up as a sortcut in Gesture Launcher but I never had any luck with that app. All it does is force close for me.
Edit: Sorry, misread you question. I don't know of any app that does that and I looked around but came up empty. Personally, that would actually be the ideal behavior of the screen orientation and while I'd have no idea how to make it, it can't be that hard. I'd recommend asking around at the XDA forum. Hopefully someone there may know or maybe one of the devs there will make an app to do it.