I just placed 2 orders on BitInstant with Dwolla and both failed.  I paid with my Dwolla balance and both transactions are <=$500.
https://www.bitinstant.com/order_status/95a1fd7d-0827-40bb-b9d7-0a33e1f42ba8 (Dwolla tx id: 1742833)
https://www.bitinstant.com/order_status/6364ce56-6042-4948-8d3d-2ebac719d10b (Dwolla tx id: 1742839)

Thanks!