Try posting here if you haven't already:
https://bitcointalk.org/index.php?topic=128314.0I don't know about anyone else but my order has failed every time I've placed a BitInstant order using Dwolla. Contacting customer support always cleared it up but sometimes it takes a couple days.
I think you'll get current market price whenever they process your order. I'd recommend using MtGox and transferring funds in using Dwolla; that way you know exactly what rate you're buying at.
Good luck!