Build Warning: Disabling subview autoresizing is not supported on iPhone OS versions prior to 3.0

This build warning just started showing up. I'm building for 3.1.3. Not sur开发者_如何学Pythone what I did to trigger it. Anyone know what it means?!?

CompileXIB Classes/RootViewController.xib
Disabling subview autoresizing is not supported on iPhone OS versions prior to 3.0.

Click the Info button on your xib's main window, and set the Deployment Target to iPhone OS 3.0 or iPhone OS 3.1.

Edit: Images!

(NB: These are from the 10.5 version of IB, but IIRC the 10.6 version is the same or close to it.)