Please elaborate further, how this could be done. We have no need for the coins before the maturity, so if there is no risk to us, or hassle to the user, we certainly want to give it a shot.
The basic idea is that funds are blocked in such a way so both service's and user's signatures are require, i.e. neither service nor user can move money on his own.
There is a risk that user won't unblock funds when he should.
Normally this isn't a problem as you can give user an incentive to unblock funds. For example, user deposits 110 BTC, puts 100 BTC into a bet, loses it. He still has 10 BTC on account, and he needs to unblock whole sum to get it.
However, there can be a problem with users who are stupid assholes, are dead or want to ruin service financially at their own expense.
I don't know how likely are these scenarios, but it's possible to try to fix it using a more advanced scheme, say, 2-of-3 signature: some trusted 3rd party can unblock funds when user isn't cooperating.
As for the tech side, in the most minimal way you can generate a private key from a user's password in browser and use it to sign multi-sign transaction. It is relatively easy to implement.
People did something like this via command-line interface... However, as far as I know, currently it isn't used in any of user-facing apps. blockchain.info has a page about escrow, but it isn't present in wallet.
In general, what you said sounds like a good way to do all kinds of betting services. People not releasing their funds, or not releasing them in a timely fashion, would be just contained in the rules of the game. Some people will probably not release their money even if they get something in return, but that could just be subtracted from the winners amounts. And in many cases the service provider would be position-neutral, and the site could be run with zero capital. I wonder why this kind of thing doesn't seem to be standard by now.
Even without the use of a mediator this kind of system seems quite good to me. The risk the user of a site like this takes would be that he wouldn't get any of his winnings, but would get his own money back. In an extreme case the operator of the site could refuse to release anybodies money, but he wouldn't get it either. The whole site could also be shut down by the authorities, with the same result.