Sorry to bump an old thread, but just figured I'd mention that I ran into the same issue and give the details in case someone who works in-depth with bitcoin has more information to work with.
Anyhow, sent a small transaction that had the status of "0/unconfirmed" followed by something about being broadcasted out through 12 nodes or something like that. Later went to just "0/unconfirmed" though.
Status: 0/unconfirmed
Date: 6/18/2012 18:33
To: 1LTnpXQHYCT5V7132AuGSW6jvnD2dSfMXk
Debit: -4.23958261 BTC
Net amount: -4.23958261 BTC
Transaction ID: 9cf62b043a484baa76d98932d7307882bbbe9ff47ab45bbd33c79ec7887a8d33
It deducted the coins but never showed up in the blockchain. Someone checked the bitcoincharts for me as well for a low priority transaction that was queued but came up with nothing.
Used pywallet to delete all the keys and rescanned (returned ~2.9BTC) as well as trying to delete the specific transactions that were unconfirmed and rescanning (put me at a balance of 0BTC). *Note: There was 1 other unconfirmed transaction besides the one quoted above.
I did have an issue about a month ago where I got a message about a double-spend when my bitcoin wallet crashed as I went to send payment for something. The double-spend message later disappeared from the blockchain as well. However, that transaction was for 4.0BTC so I'm not sure why fixing my wallet gave me ~2.9BTC
I've had 90 successful transactions and 2 bad ones within the past 30 day period.