After a short period of testing this, I've found that a mailto: link only seems to accept a character limit of 500. I've Google it and found a number of 2000 should be more the case? One work around would be to put some of the addresses into the 'CC' field I suppose, but not as elegant as I would like.
So where is the limit? The browser? The e-mail client? Or is it an RFC agreed limit? Anyone got any hard facts on this one?