Glad to hear it got resolved in a good way, to be clear an invalid bitcoin address is one where the check sum doesn't add up correctly (no private keys exist for them) & it is impossible to send to these so funds can never go in to the void that way, what you were referring to was an old deposit address that was used in error - in the case of Mt. Gox for example they issue new ones to you for each transaction (for privacy) but you can still actually use an old one of these & it will work fine, it just depends on how the service you are using set their systems up.
What you don't want to do is send funds to a valid btc address where the private key is unknown or has been discarded then those funds will be stuck there & are effectively lost unless the private key for that address is subsequently found to not have been irrevocably deleted from where ever it had been stored.
http://blockexplorer.com/address/1BitcoinEaterAddressDontSendf59kuE <--- valid btc address with unknown private key = lost
http://blockexplorer.com/address/1AnyOldBolloxWithInvalidCharsOIl0OI <--- invalid btc address with invalid chars: OIl0 = can't send to
http://blockexplorer.com/address/1BitcoinEaterAddressDontSendf59kuF <--- invalid btc address, check sum fail = can't send to
