Sen. Barack Obama decided to forgo public financing, under this system, candidates agree to spend only the public funds and cannot raise or spend money directly obtained from individuals.

http://www.cnn.com/2008/POLITICS/06/19/obama.public.financing/index.html

Also, his campaign is allowing donors to use largely untraceable prepaid credit cards that could potentially be used to evade limits on how much an individual is legally allowed to give or to mask a contributor's identity.

http://www.washingtonpost.com/wp-dyn/content/article/2008/10/28/AR2008102803413_pf.html