I don't know if it applies to plasma screens, but any glass cleaner that contains ammonia is very bad for LCD screens. Most glass cleaners, including Windex, are ammonia-based. I wouldn't risk it on any sort of display, unless it's a good old CRT. I use a microfiber cloth, dampened with a little water, to clean off my screens (laptops LCD's, DLP TV, etc). I know that the manual for my LED DLP HDTV (Samsung) specifically warns against using any sort of glass cleaner on the screen.
I have a bottle of LCD/DLP screen cleaner that I use if there's a smudge on it. A small investment to protect expensive equipment.
I have first-hand knowledge of what repeated cleanings with Windex do to LCD monitors. It's not pretty. First it discolors the screen. And eventually it can even peel off the coatings. Either way, it ruins them.
At my job, we have a factory area that generates a lot of dust. There are LCD monitors in those areas that have to be cleaned regularly. Even though we (the IT dept) had told the folks that use those monitors to *not* use Windex to clean their monitors, they did it anyway. The results were completely unusable monitors in about 6 months. Imagine smearing your monitor with butter. That's about what it looked like when the users finally complained that their monitors had 'gone bad'.
