Greg Bilsland (who is always a great bloke to deal with) just tweeted this little gem, presumably in relation to the 2nd Rise of the Underdark Lair Assaults, Kill The Wizard:
@GregBilsland: Preparing to playtest a Lair Assault that’ll be coming late this year. Need to read up on my flying rules. #dnd #lairassault
It seems now that it wasnt in relation to Kill The Wizard, but the following Lair Assault (Temple of the Sky God) which will include flying mounts!