Mr. Answer Man says: Sadly, it means he's not ready for a serious relationship with you.
Think of it this way: if he saw a $100 bill on the sidewalk, would he be "ready" to pick it up? You only want to get serious with a man who thinks you're gold; anything less and you're asking for heartbreak.
The good news however is that, though he might not be ready for a serious relationship, at least he's being honest. There are lots of guys out there who would let you think things were serious while keeping an eye out for something better.
So now, the choice is yours. You can just cut bait -- he's not ready, you want more, so let's find Mr. Ready -- or you can accept the situation for what it is and keep things light. Which is something to consider. "Not serious" relationships can be serious fun.
Go to the Guy Talk message board to find out what other real men say about "not being ready."