jamie found an article over at Washington Monthly discussing the recent finding that there is no link between thimerosal and autism. It seems that after the mercury-based vaccine preservative was withdrawn from use in 1999, no drop in autism rates has been observed in a large California study. Here's the Science Daily writeup on the study, published in the Archives of General Psychiatry.
The GPL doesn't say that you can't create derivative works without giving them away. It says you can't
*distribute* derivative works without giving them
If you're building proprietary software for use internally by the company, then they are not, legally, distributing it! In effect, it is purely free, so long as you don't distribute your derivative works.