What function does Mt. Gox's "Open Transaction" withdrawal option perform?
I just asked on the #mtgox IRC channel, and was told:

06:36 &lt; dooglus&gt; what does the "Open Transaction (6 Confirmations)" option
                 do when withdrawing BTC?
06:48 &lt; Cory&gt; If the Bitcoin address you're withdrawing to is a MtGox address,
              it will use the network and blockchain anyway.
06:49 &lt; Cory&gt; Otherwise, if that option is not checked, they will only internally
              transfer the funds.
06:53 &lt;@MagicalTux&gt; and instantly


So there you have it.

I suggested they should label it differently:

07:13 &lt; dooglus&gt; it would be better to use some different text for that option -
                 like "use bitcoin network even for transfers to mtgox deposit
                 addresses" for instance
07:19 &lt;@MagicalTux&gt; dooglus: it use to be something like that, but people were
                    complaining it was not clear enough