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You really need to take the job more seriously if you're going to handle other people's money. What no one wants to hear is, "oops, I just sent some/all of the BTC to the wrong person" (or "the wallet.dat with your bitcoins got corrupted" etc.) "and can't afford to replace it. Sorry, I never knew that was even possible"...

Well, I think you misinterpreted events a little bit, since they are really far from "oops, I just sent some/all of the BTC to the wrong person". Actually not even "Sorry, I never knew that was even possible".
We did know this was possible, the only checks on addresses we did were for the proper number of characters and proper set of characters, because we assumed that well, if you are going to let us handle your money, you will at least make sure to enter right addresses for the payouts.
Unless the users contact us and we will discover that our service somehow tried to pay to a wrong address even though the user did enter the right one. We have checked both logs and the code looking for such possibility, but have not found anything. Having said that, no software in the world is bug-free and everything is possible, so if that will be shown to be the case, we apologize.

However, as of now no money has been lost, nor were any money of other users ever in any danger because of this, the service is operational as usual, we even redeemed the balances of users that did this. You think that we should have done something differently?

Like I said, we will implement the checksum check, but that is not really on top of the priority list, since it is just a small usability thing, with no security implications.

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Here's PHP code I wrote to do address verification (and other things):
http://pastebin.com/vmRQC7ha

Bitcoin also has a validateaddress JSON command.

Great! For security reasons the front-end of our solution cannot call bitcoind directly, but the offline version based on your code should work fine.
On a different topic, theymos, you seem to know enough about bitcoin to be Satoshi Tongue

Even saw your signature in blockexplorer errors, great job there!