As Josh mentioned, as long as the MinimumOSVersion isn't set above 5.1.1, it might be that an iPad 1 could still install it from the app store, based on Apple's official documentation (unless that is out of date).
Though who knows if Apple rejects any new submissions if MinimumOSVersion is below 6.x, but it may still work yes. I guess it would be pretty easy to test it if you have a new app in the works and want to try setting MinimumOSVersion to 5.1.1
I can say for fact though, that we just a week or so ago went live with a newly released AIR app that had MinimumOSVersion set to just 6.0 (not Adobe's official 6.1 supported version from that link above), built with AIR 17, and from the Flox data we definitely have users running and installing the app on iOS 6.0 and 6.0.1, so I don't see a reason why it isn't still possible for an iOS 5.1.1 iPad1 user to get an AIR app if built with compatible MinimumOSVersion set.
I guess if someone is planning on releasing a new AIR app built with 16+ SDK and use a MinimumOSVersion set to 5.1.1 it would be interesting to let us know if they see iPad1 users from their analytics data... It might even be possible checking in iTunes Connect after submitting the binary should tell you what compatible devices Apple will show in the store.