There's already a Hulu app for iOS. The only reason they'd have for blocking AirPlay would be if the content providers got bitchy about it - but the thing about AppleTV is that it's all streaming. This makes it, conceptually, no different than streaming the content to a browser or to the Hulu app itself like they already do. The AppleTV is a closed box with no apps, so Apple can claim to the networks that it's very secure, etc. The content providers might be more open to that idea since the data itself is always transient (streamed from the net to the app, from the app to the TV - nothing is ever stored which is what the providers are deathly afraid of).

The other angle here is that this could eliminate the need for Hulu entirely. Smart networks could make their own apps that follow their own desired content rules (letting you see live, non-live, only the last X days, whatever they want). And they can stream the content to the app in whatever manner of subscription they choose. And indie production companies could skip networks entirely and go straight to the App Store - just like indie musicians and podcasters are already doing.