TPM Muckraker has the dirt:

As reported today by the AP and the Philadelphia Inquirer, the Green Party managed to get their candidate Carl Romanelli on the ballot with a costly petition drive, which was mostly funded by contributors who had also given to Rick Santorum's campaign. The party raised $66,000 for the effort, all of which they spent on a private company to collect signatures. TPMmuckraker was able to establish that at least $55,000 of that came from conservatives.

Virginia Davis, Santorum's spokeswoman, told the Inquirer that their office had encouraged the contributions. Why? Because a challenge from the left is seen as a liability for the Dem candidate, Bob Casey.