Is butterfly weed invasive in the Northwest?
First, I have a plant name question for you: did you mean the shrub Buddleia davidii which is also called butterfly bush, or the herbaceous perennial Asclepias tuberosa, the plant most often referred to by the common names butterfly weed or butterfly milkweed?
In King County, Buddleia davidii is a Class B noxious weed, which means that its control is recommended but not required by law. Many gardeners grow this shrub as well as other less aggressive species. If you absolutely cannot live without this plant in your garden, you can go a long way toward preventing its spread by keeping the blooms deadheaded as soon as they fade (before they set seed).
Asclepias tuberosa can seed itself around, but it is classified as an endangered plant in New Hampshire, a threatened plant in Vermont, exploitably vulnerable in New York, of special concern in Rhode Island, and possibly extirpated in Maine. It attracts butterflies and hummingbirds. Here is more information so you can determine which plant you have in your garden:
Lady Bird Johnson Wildflower Center
USDA Plants Database