Aboroth,
You have to call Onvia. That was exactly what has happened to me. My order was showing back ordered as well, and I called in to cancel it. Then, they told me that they turned out to be in stock. Now, my order status is showing item shipped.
My take of what happened was that they probably assign one warehouse for your order. When you place your order, they look for all of their warehouses for the available inventory. If the item is backordered at all of thm, they assign one closest to your address and wait for the item to become available at that particular warehouse. The problem is that the item can get stocked in some, but not all, of the warehouses. If this happens, your order will not be shipped unless you call them and have them re-assign a warehouse.
This has happened to me several times with Onvia. I had to call them for IBM 75GXP hard drive, Pentium 700, Linksys Router, this slocket, case fans and so on.
So make sure to call them for yours.