I've been pondering Slaty-backed Gull for a while now and really think this one is due - their rate of occurence across the US eastern seaboard is staggering and probably represents the tip of the iceberg. I'll even put my neck out and say it will occur in 2008!!
Sibley has some amazing "what are the odds" stories including his finding of 2 slaty-backeds this month
Good luck all you gullers in 08