You have a good boss, and you probably joined earlier than others. However, I do have to admit that it's as good as it gets, in the big corporate world.
I suspect that my experience is different from others more in the degree that it's officially sanctioned, not in what actually goes on though. I've heard - from several friends across several other big companies - that it's really common to have everyone spend about two hours a day working on their "real" job, and then the rest of the time either reading SlashDot or working on side businesses. After all, what's the worst they can do, fire you? Then you're in the same boat as someone who quits to do a startup, except that you can possibly collect unemployment. Most big companies are large enough that nobody knows what anyone else is really doing, anyways.
Fire you, and keep the stuff you made on company time.
On the other hand, people have been talking about launching early and often. Even your first version of something could well be good enough to launch and iterate upon.
All in all, I'd rather not give my employer any of my own stuff.