Apple claimed the S stood for Speed, and when Apple released the iPhone 3GS it was manly just an internal hardware improvement with no physical changes, so with many rumors floating around that Apple is only going to be improving the internal hardware and not making any physical changes from the current iPhone 4, the internet has decided to call this device the iPhone 4S (to signify no external changes, only an internal upgrade).
The iPhone 4 Plus just implies that its an improvement to the current iPhone 4 with a few spec bumps (a lose of GeeBee's but a gain of an A5 processor is still an upgrade).