The presently available materials on Red Ryder are unclear, but Toady definitely survives solely on donations [http://www.kwanzoo.com/social-trivia/tarn-adams-interview-pa...]
Randy Mulholland of the webcomic Something Positive quit his job when he received in a month donations that exceeded his yearly salary.
Most webcomics that are 'full time jobs' for the artist also sell t-shirts, taking them a slight step outside of 'pure' donationware, but including these artists, the numbers of successful donationware vendors go up.