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.

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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.)