I think getting a text like "I support CF" under the title would be pretty good.
Wouldn't make the forums look unproffessional like pictures in sigs would, (just my opinion) and new members would know why people have the text under their title, if it was a custom title (like u can type anything u want in it) new guys would probably be wondering why some people have it and why some don't. They would be making threads about how they can get one etc...
just my 2 cents, and I'm most likely not going to donate anything, since I feel like I'm contributing enough by helping other people, and also the ads should give David enough money to pay for the server.