is time out worth it?

K. is now 18 months old.  I have no idea when my sweet little infant turned into a running while simultaneously chewing on his shirt tail boy, but he did.  He also has started to be a bit more sure of what he wants.  This is great when it is something that he can have.  But, lets just say it is the toy that A. was peacefully playing with or the blender where I'm dropping the fruit for our morning smoothies.  Then, when I say no, it isn't so peaceful.  Recently, it has turned into running away from me with his mouth open and hand cocked back ready to hit and/or bite his sister.  Yes, she could be in a totally different room or sometimes even on a different story.  The best thing about this is, after he completes his action of choice, he puts himself in timeout.  Sometimes I say his name, sometimes I point, other times I just find him sitting in the corner and I have to ask A. what happened.  She almost always answers with, "he hit me."  He sits for a minute, stands and rubs his chest to "sign" sorry, gives a hug and then goes on about his business.  The scenario starts over again sometimes as little as a minute later.  So, obviously, this is not working.  He either decides that hitting his sister is totally worth the time out or he just doesn't care.

no, this did not result in timeout, I just thought he looked particularly devilish

So, I'm ready for some advice.  How do I get my kid to decide that timeout is not fun and hitting is not worth a timeout?