Homeless signs on tumblr is not like that time GoGorilla used homeless signs as adspace, or like when pizza was advertised by homeless people paid in pizza, nor is it like those winter jackets advertising Ben&Jerry's given out to the homeless. Nor is it as cynical as when homeless people held up movie posters in Hollywood. Instead it's a artists self-promo piece, sort of. Artists Kenju Nakayama & Christopher Hope * give the homeless people re-designed signs (with the same message) in exchange for the old sign and $10, also do a short tumblr interview with each person. The new sign is so typographically hip that if good design alone "sold" things, the homeless people should see a large uptick in donations. But like that one weird trick, the unprofessional look of their signs might be exactly why they work.
Like the homeless hospots, which in at least one case helped a person save enough to get off the streets, homelessness is a touchy area, and appearing to use the already marginalized people for your own gain may very well backfire on the artists even if their intentions are nothing but good. See more of Kenju Nakayama's skilled signs and other work here.