I apologize for the double post above, sometimes happens. I live in California and am probably the last person that could comment about how common a particular structual personality is in a New England brownstone or what it would cost to fix in that market. I am fairly certain others will comment, on the cost issue and how frequenty they as contractors observe this problem; so stay tuned and there may be some interesting things to learn.
Congratulations on the house. Having family in Boston, I know how desirable those historic properties can be. Good luck Andrew, and sounds like you'll be a regular aroung here.