#5 - Aug. 26, 2009, 5:20 p.m.
While heirloom items are considered Bind on Account, they may only be transferred through our in-game mailing system at this time. Because of this, they're beholden to the same restrictions that normal mail is: can't be sent cross-faction, can't be sent cross-realm, etc.
With that said, if a character is transferred with an heirloom item in tow, that character would then be able to share it with any other characters on the account of the same realm and faction. :)
Here's a bit more information:
http://forums.worldofwarcraft.com/thread.html?topicId=11296528267&sid=1