The 'Alliance for Community Trees' looks like a great place to get involved with urban forestry. This is what they say about themselves:

"The Alliance for Community Trees (ACT) is dedicated to improving the environment where 83% of Americans live - cities, towns and metropolitan areas. Together, local ACT member organizations have planted 7.8 million trees with help from 450,000 volunteers nationwide.

Mission Statement

To support grassroots, citizen-based nonprofit organizations dedicated to urban and community tree planting, care, conservation, and education."

Go guerrillas!!! (see post below).