You might try slaving your hard drive to a second computer and using some file recovery software to try to recover the wallet.dat. Of course, I would advise immediately ceasing use of the computer (just powering it off with the power button would be better than shutting down, lest the wallet file be overwritten by some files that are written during shutdown). A decent piece of software might cost $50.
If that doesn't work, there are professional labs that will read the actual data on the disk, if it hasn't already been overwritten. A decent lab would cost $1000 or so. Don't cheap out on this if you try it. I did, and I regret it, a lot.
And beyond that, if you use a mechanical hard drive, forensics labs can actually tell what bits were written to a given sector BEFORE the current bits, because the previous bits leave some sort of magnetic bias behind. That's why military erasing standards dictate that a drive must be overwritten 7 times with random bits before the data that was on it is considered securely erased. I have no idea what it might cost to get a forensics expert to look at your drive though.
If that doesn't work, there are professional labs that will read the actual data on the disk, if it hasn't already been overwritten. A decent lab would cost $1000 or so. Don't cheap out on this if you try it. I did, and I regret it, a lot.
And beyond that, if you use a mechanical hard drive, forensics labs can actually tell what bits were written to a given sector BEFORE the current bits, because the previous bits leave some sort of magnetic bias behind. That's why military erasing standards dictate that a drive must be overwritten 7 times with random bits before the data that was on it is considered securely erased. I have no idea what it might cost to get a forensics expert to look at your drive though.
Unfortunately, I securely deleted the plain text wallet before I realized there was an issue, so it was probably overwritten many times with random bits. I was basically following the steps I found in a thread describing how to create a secure savings wallet.
Even if you can't recover the Private-key for the receiving address, keep both your wallet backups safe and don't give-up and delete them. The BTCs might still be unclaimed by the address that you lost, maybe in the future the network rules will be changed to allow old (1 year, 6 months) unclaimed transactions to be reversed. I really think this should be possible. It will require a change to most of the bitcoin clients on the network before any block containing a "return unclaimed BTCs" transaction will be accepted.
Lookup the sending/receiving address on http://blockexplorer.com/ and find the transaction/block that contains the lost BTCs.
Also I think that new wallets should default to returning change back to the sending address, let people that want anonymous transactions to change to the current method of sending change to a new address.