I think the best that this can be answered by anyone outside of the intellegence community is that back doors into closed operating systems and software may exist but will be hard to know about unless leaked or discovereed by security researchers, whereas doors into open code bases such as GNU/Linux have to get past the eyes of the many who review the code, at least some of whom might qualify as not being supporters of the NSA.
Most of us use PCs that have proprietary BIOS firmware, so a door there might be difficult to know about, no matter how open the OS. One would hope that such an opening would be eventually discovered, if only by accident.
Desktop 1: DFI Lanparty UT nF4 SLI-D/AMD Athlon 64 San Diego 2.2 GHz/G.SKILL 1GB x 2 DDR 3200/BFG GeForce 7800 GT/Ubuntu10.04
Desktop 2: Gigabyte 770T-USB3/AMD Phenom II X4 965 3.4 GHz/s. Talent 2GB x 2 DDR3 1333/EVGA GeForce 9800 GT/Mint 14 Cinn-lowlat