Employers: Don't Panic!
If you develop free software, you and I may have corresponded. As the Copyright Administrator here at the Free Software Foundation, it is my job to make sure that we have the resources we need in place to protect the GNU General Public License. Not everyone, after all, is as ethical as you or I when it comes to source code.
Some people don't see contributing to free software as a kind of contribution to humanity - to the right to learn from those who came before. I maintain the records that prove there are thousands of developers from every part of the globe that have contributed to the GNU project. That's a lot of contributors...
The most common problem I face each day is receiving the rights to a developer's work on free software from employers who see no reason why they should give the foundation copyright to THEIR employees' work. Many employees sign agreements with their employers stating that that what copyrightable work the employees may produce ought to belong to the employer. It's important for employees to be certain. Sometimes, through the "work for hire" doctrine, an employee's work will automatically belong to the employer.
What interest a corporation might have in retaining the rights to 100 lines of EMACS code when there are over 600,000 lines of code  identified and assigned to the Free Software Foundation is mystifying to me.
Most employers are rational, however, and with sufficient explanation understand that allowing contributions to the GNU project actually does little damage to the bottom line. Corporations are not opposed to learning, or to expanding utility.
What's more, many of the more intellectual, or more ethical, developers, will not work for an employer that will not allow contributions to GPL protected projects. Arguably, intellectual and ethical employees are desirable enough that an agreement to disclaim its copyrights to an employee's contributions, in fact, helps improve an employer's bottom line, even as it helps their recruitment and retention efforts.
Free Software, what with its requirements to treat computer users fairly, helps spread work across a vast community, and improves the bottom line for everyone - financially, sure, but most importantly, morally.
But that's not my argument to make. Developers working all across the world, make that argument everyday, for themselves.
Be talking to you,
(WORKING WITH FREELANCERS:WHAT EVERY PUBLISHER SHOULD KNOW ABOUT THE "WORK FOR HIRE" DOCTRINE; Attorney Lloyd J. Jassin (c) 1997 - 2000. Lloyd J. Jassin. All Rights Reserved. http://copylaw.com/new_articles/wfh.html
 (More Than a Gigabuck: Estimating GNU/Linux's Size; David A. Wheeler (email@example.com) June 30, 2001 (updated July 29, 2002)Version 1.07 http://www.dwheeler.com/sloc/redhat71-v1/redhat71sloc.html)