Appears that as long as you never “distribute” a program (for instance, a program that runs on *your* web server), then it appears that you *can* mix GPL with proprietary software/closed source, etc.  You just can’t release for instance the binaries without (possibly) having to release it all as GPL

refs: http://stackoverflow.com/a/94496/32453

http://stackoverflow.com/a/1893597/32453 “in-house” is ok

http://blog.milkingthegnu.org/2008/04/gpl-for-dummies.html ”as long as I don’t redistribute it”