There's nothing to stop you from posting the answer, so people will see it for the two days before you're allowed to accept it.
Eric and Jonathan's points are valid, but sometimes I agree it would be nice to be able to accept your own answer immediately - because it's obviously right. As an example of this, look at my Code Contracts question about arithmetic: I'd asked on the MSDN forum as well, and one of the CC team answered the question. I checked the answer, it worked, I reworded and reposted it on SO.
It's clearly right, so it would make sense to be able to accept it immediately... in this particular case. However, I suspect that the general principle of "let the community warn you of problems first" is more important than specific cases where immediate acceptance would make sense - so I support the policy overall.