Last week, Cisco filed a lawsuit charging Apple with trademark infringement since Apple introduced a mobile phone called the iPhone.

Hemel didn't actually identify for Cisco the specific code that hasn't been published. "I'm not going to do their work for them," he said. He suspects that a large company like Cisco might hire various programmers possibly from outsourced companies around the globe in order to create a product like the iPhone. That might make it difficult and potentially expensive for Cisco to properly document and account for all the code in the phone, he said.

Cisco representatives did not immediately reply to phone calls and e-mails made on Wednesday asking for comment.

If Cisco is violating the terms of the GPL license in the iPhone, it certainly isn't alone. "It occurs more frequently than we'd like to see," said Shane Coughlan, Freedom Task Force coordinator for the Free Software Foundation Europe. The problem is that many organizations don't quite fully understand the concept of free software and often don't have appropriate policies that enable them to comply with their software licensing agreements, he said.

"If you're used to buying code and used to owning it, it's difficult to understand having code that is owned by thousands," he said. "A lot of companies have been good in trying to come into compliance and figure out how sustainable compliance can be introduced in company policy."

There are repercussions to failing to comply with an open-source license. The GPL Violations Project has successfully enforced 100 violations.

In addition, an individual who contributed to software that someone else fails to properly use under a license can take the licensee to court and look for financial compensation for copyright violations, said Coughlan.