Companies buy out competitors all of the time. That's how you eliminate competition and it usually works out that the person being bought out is a competitor. This is just basic business sense. These companies aren't going to attract much attention from the SEC, just because they would have a dominant position in the videoconferencing area. Whatever happens, it's a buy from here.
I will agree that I think PLCM is poised to be bought, but there is no WAY CSCO is the buyer. SEC would NEVER allow it. Maybe they don't care when Taco Bell buys Taco Casa, but I am POSITIVE they would notice at this level.