I am writing software that will be used ONLY internally in a corporate environment, and the company will never market the software or make it available outside it's own confines. If I use open source (i.e. GPL) code in my application, does the company have any obligation to release what I write? Or if it's never marketed can we keep things "secret"?
The reason for this is that the applications themselves aren't so special, but the domain knowledge they will contain are.
If I use open source (i.e. GPL) code in my application, does the company have any obligation to release what I write?
Absolutely not. Keep it as secret as you want. The GPL is a distribution license.
My Lawyer said you're ok.
Edit: By the way the "secret" part is not necessary, you aren't hiding it in a sense, just not releasing to the public.
Paul Betts is correct.
See this quote from the GNU FAQ
Does the GPL require that source code of modified versions be posted to the public?
The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.
But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL.
Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.
You can still make your software public without marketing it. For example, a patent application for the software would count as a public release.