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
Hungry Pests website for most wanted USDA APHIS regulated pests
Beetle Busters (Asian longhorned beetle only)
Canadian Food Inspection Agency regional offices (they also cover plants and plant pests)
This page and the links are accurate as of September 2012. Please note that these things change over time, and this is only a good-faith effort to help you out!