The back button in Android is supposed to mean "take me to the last screen I was on"; it's not supposed to be tied to the specific app you're in. I have the reverse issue on iOS, if Twitter launches a browser there's no obvious way to get back to the screen I was on in Twitter beyond calling up the app switcher[1] Honeycomb introduces an "up" button[2], and it can be presumed the concept will make it's way to ICS.

[1] PSA: you call up the Android switcher by holding Home

[2] This isn't a hard button like home/back, instead the app logo gets painted with an "up" arrow when this is available.