Loosely speaking, the GPL requires that you offer to make the source code available to whoever you make the binary available to. If the application is only for internal use, then this is probably not a problem, since you are presumably not worried about your internal users requesting or using the source.
Edit: Note that, to comply with the GPL, you're still obligated to offer the source code (even if no one takes you up on your offer), and you could conceivably get into dicey territory if an internal user insists on getting a copy of your source and you're not ready to give them one.
Edit: I did not realize that the GPL FAQ specifically excludes internal use from being considered distribution, which makes David Thornley's answer much better than mine. I guess I'll leave my answer since it covers the broader issue of limited distribution.