The Center for Responsive Politics has issued this interesting press release, which notes:

The tax code provision at issue was included in legislation signed into law in 2000 that requires groups organized under code section 527 to file regular reports with the IRS that detail their contributions and expenditures.
These groups must identify the name and address of contributors of $200 or more and recipients of $500 or more. If the donor or recipient is a person, the group is supposed to list the employer and occupation of that individual.
However, section 527(j) of the tax code, as amended in 2000, allows groups to omit disclosure information as long as they pay a tax on the amount not disclosed. 527 groups do not pay taxes on income that is properly disclosed.