From his Twitter, as it happened just minutes ago:
No reason given yet for the emergency landing.Currently emergency landing in Albany. Rad.
Firetrucks awaiting our arrival http://imgur.com/3RpbGTw
This is @AmericanAir flight 3364 from LGA to YUL
Safe landing. More firetrucks. http://imgur.com/1wnYZmb
Leaving our belongings on the plane and deplaning
Deplaned no stairs no slide. Firefighters helped us jump down. http://imgur.com/3LmlOrn
Six emergency vehicles now
I would be freaked out if this were happening, unless the emergency landing was over some unruly passenger or something else non-threatening.