Just read the latest posts below, it seems USB_OTG is still unconfirmed 100% but latest official info seems to be saying it doesn't work with external storage devices. 
Still glad I ordered one though, this wasn't a deal breaker for me, but would've been nice. Have it on the Galaxy S2 but only used it once.
AnandTech have just confirmed that USB_OTG is available on the Nexus 7 Google IO version.

Scroll to the bottom of the review
AnandTech - Google Nexus 7 and Android 4.1 - Mini Review

Still glad I ordered one though, this wasn't a deal breaker for me, but would've been nice. Have it on the Galaxy S2 but only used it once.
AnandTech have just confirmed that USB_OTG is available on the Nexus 7 Google IO version.


Scroll to the bottom of the review
AnandTech - Google Nexus 7 and Android 4.1 - Mini Review
