That's on the specially-treated against graffiti wall of Shepard Fairey, who is of course best known for ripping off an AP photo to create the most iconic political image of 2008 - and the most ironic one of 2009 (not to mention, one of the most parodied). For extra humor, Fairey likes to do precisely this kind of guerrilla art himself... and he's probably swearing at Big Government's Best. Weblaunch. EVAR. right now anyway, which is a definite bonus.
Moving back to the main point, there's at least one other tagging example (see BG's link above for it); my guess is that they're using a stencil and some spray paint. As a law-abiding Republican I of course heartily wave smy finger* in the direction of whoever is doing such naughty things - bad street artists! Bad! No biscuit! - and I am explicitly shaking my head and intoning that this sort of thing not be duplicated...
*Admittedly, I know that I'll get it back: wait, no, that's MoveOn.org. Ach, well, these people all live in each others' back pockets anyway.
Crossposted to Moe Lane.