Are you concerned that you've discovered a new infestation of a forest pest or pathogen? Did you find a strange bug, or do your trees look sickly in a way that you've never seen before?
USDA APHIS Report a Pest map
http://www.aphis.usda.gov/services/report_pest_disease/report_pest_disease.shtml
Complete list of State Plant Health Directors
http://pest.ceris.purdue.edu/directory.php?list=SPHD
Asian longhorned beetle contacts
New England: 1- 866- 702- 9938
Illinois, New York, New Jersey and more: http://www.aphis.usda.gov/newsroom/hot_issues/alb/alb_report.shtml
Canadian Food Inspection Agency (they also cover plants and plant pests)
http://www.inspection.gc.ca/english/directory/offbure.shtml
Canada's Asian longhorned beetle hotline
1-800-442-2342
This page and the links are accurate as of May 22nd, 2009. Please note that these things change over time, and this is only a good-faith effort to help you out!