I don't know of a convention per se, but there 
will be an informal gathering of cel collectors 
next Saturday (5/19) at Koreana in Cambridge 
starting at 12:30.

You can get a little more information from the 
discussion list at NEACC
  (you may need to register first before that 
link works)

NEACC - New England Anime Cel Collectors

---Trebuchet