How to withdraw dollars to Dwolla from Mt Gox
Apparently there is no way to know if a person has a pending Dwolla transfer for funds exiting Mt Gox.  The most confusing part about this is the pending status remains pending in Mt Gox for at least a day after Dwolla shows the transaction as having completed.

So I recommend waiting an extra day after Dwolla says the funds have transferred.  Otherwise the transfer will be canceled with no notification from Mt Gox or anyone else, and no reason is given for the transfer cancellation.  In fact, the only indication that the transfer is canceled when this happens is that the balance returns to the pre-transfer balance, and the funds never show up in Dwolla.

To Mt Gox's credit, Mt Gox did respond to my support requests.  However, getting a satisfactory response took almost two weeks of asking combined with plenty of trial and error with the Mt Gox withdraw system.