I don't know if you would actually call it "Guerrilla Gardening" or not. But I did gift a flat of Impatiens and planted them in the front flower box at Burien Press, in honor of their third anniversary. Just because they are nice people and great for the community. They always are donating coffee for public events, charity auxtions, etc., and their coffee shop has become a central location for people to meet, talk and take a rest. Add to that they are very important to the local art scene with shows and such right in their place of business.
Nice thing is the owner said he will water them over the summer. And I don't have to worry about the city removing them!