It looks like they're already testing iOS 6 on the iPad: http://www.idownloadblog.com/2012/03/02/ipads-running-ios-6-0/
iOS 6 isn't in beta yet. We all know how long that takes, every OS is at least started 1 year before release, planned probably 2 years before.
I'm sure they're pretty much done with iOS 5.1 and should release it soon... iOS 6 is barely in Alpha testing. In a couple of months they should release the Dev preview for Beta testing.
Read those statements in that order. Apple could very well be testing iOS 6 out on the iPad, but this could be the beginning of planning.
Yeah, 5.1 should come very soon, (heck, may even launch on the event day,) and the iPad 3 may come with iOS 6... But, that won't launch for a considerable amount of time, where it would then have the exclusive until the iPhone 5 comes out, with iOS 6, so it's released on all of the other devices. (Because, Apple wouldn't release it early and accidentally leak features of the iPhone 5.) It's possible. iDevices before have been late or early to iOS firmware versions. (Original iPad getting iOS 5 I believe, and the Verizon iPhone 4 having one or two numbers higher than everything else before iOS 5 I believe, etc.) But I doubt it's coming out soon.
i wouldn't be surprised if apple releases a beta of iOS 6 earlier then usual considering they also released the OS X beta earlier then usual.