Bitcoin's security is based on the idea of a "block chain", which is sort of a mutually agreed-upon list of transactions. As long as all participants in the Bitcoin economy are using the same block chain, then double-spending money is (for all practical purposes) impossible.

It was recently discovered that a change in Bitcoin 0.8 caused an entry in the block chain that old versions can't process. The old versions, unable to verify the 0.8 block chain, automatically started their own block chain based on the last block they could deal with.

  0.8-readable block chain continues
  |
  ---> |
       |  | <- 0.7-readable block chain splits off
       | /
       |/
       *  <- block that old versions can't process
       |
       |
  (history)

For technical reasons, it was decided that the 0.8 block chain would be abandoned, and all clients should switch to the "forked" block chain that was accidentally created by old clients.

  dead    | <- 0.7-readable block chain is now "official"
  |       |
  ---> _  |
       |  |
       | /
       |/
       *  <- block that old versions can't process
       |
       |
  (history)

This causes a problem, because until that switch happens, a person could issue a transaction to a 0.8 client that would be accepted only by clients running the "bad" 0.8 blockchain.

The OP states that he has done this successfully. He submitted a transaction to a BTC<->USD service running 0.8. The service, checking the bad block chain, thought it had received Bitcoins and therefore sent him USD. But his transaction was never sent to the "official" block chain, so according to the rest of the Bitcoin world, he still has his original Bitcoins.

This is significant because prevention of double-spending is a fundamental feature of Bitcoin. If it can be bypassed, then the entire Bitcoin economy is endangered.