Here's a n00b question from a Mt Gox semi-n00b, just to confirm:
If I make a BTC transaction from my Mt Gox a/c to a Bitcoin address that happens to match another Mt Gox a/c, Mt Gox simply transfers the funds "internally" to that a/c without making a transaction on the BTC blockchain unless I specifically select the "Open transaction (6 confirmations)" option? Or is this just bunk, and you have to use redemption codes to make Gox-Gox transfers of funds? Not that it matters a huge deal, just curious.
You can withdraw bitcoins from your Mt. Gox account and if the address is the deposit address for another Mt. Gox account then nothing will appear in the blockchain.
Obviously, for over-the-counter trading where the blockchain is used for evidence of having made a payment should there end up being a dispute, this didn't work 100% of the time, so Mt. Gox added a checkbox labeled "Open Transaction". When you withdraw and you mark that checkbox, then the transaction will always be sent on the blockchain, regardless if the recipient of the payment also uses Mt. Gox.
So you had it correct -- mark the block "Open Transaction" if you are using your Mt. Gox EWallet account as a bitcoin wallet and might want to track the withdrawal transaction using the blockchain.
The redeemable code / voucher is another method of doing account-to-account transfers but because they are bearer codes (redeemable by anyone who knows the code), they might not be the best tool for when performing over-the-counter trading.