As with most invasives, it's where it grows that can often determine if it's invasive.* I did a google with the term:
pampas grass + invasive + Pennsylvania
(I think I remember you live in Pennsylvania) and it appears to be invasive there as it has escaped cultivation.* It's a big problem in California and probably other warm climates as well if it's invasive in Pa.* This Pa site lists it.
http://www.murrysville.com/documents...ive_plants.pdf
I think I also remember reading somewhere that once it's established it's difficult to dig out if you don't want it anymore.
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.