Buying Bitcoins through Dwolla
Dwolla imposed a 30-day probationary period for new accounts. After your first funds have been added to Dwolla from your bank account, you then cannot trade those funds with a Bitcoin exchange for 30 days.
Dwolla did that to prevent hackers or other scammers from getting cash out of an innocent person's bank account before there is time for the individual to notice.
There are a lot of individuals who are not considered bitcoin exchanges who will gladly trade their Mt. Gox USD funds for your Dwolla USD funds:
If you want an alternate method, there are numerous. Since you are using Dwolla, then presumably you are in the U.S.
BitMe accepts cash deposited through Chase:
The next fastest and convenient method is BitInstant:
But once you are through the 30-day Dwolla probationary period, then Dwolla works pretty well for moving funds to and from the exchanges. Camp BX, Intersango, and Mt. Gox all accept Dwolla USDs for their market exhanges. Bit-Instant accepts Dwolla for direct purchases of bitcoins.