NYTimes.com:
"Invoking a provision that had rarely, if ever, been enforced, the Internal Revenue Service said it had sent letters to five donors, who were not identified, informing them that their contributions may be subject to gift taxes depending on whether the donations exceeded limits under the tax laws.
These advocacy groups have been drawing more scrutiny, from President Obama as well as others, as they have proliferated and funneled vast sums of money in support of campaigns and causes, without having to publicly disclose their donors."