Are you using the USB adapter that came with the N900 or another one? It matters.

Have you tried plugging in the USB adapter to a different USB port on the computer you are using?

I am focusing on the USB adapter and port because the only problem I see is that when you plug in the USB while holding down the u (flashing from Linux is the most reliable way to go, in my experience) you don't ever see the USB logo on the right upper part of your N900 screen. This indicates that the N900 never sees a USB connection.

When I did this, by the way, I found that the N900 screen was exceedingly dim at that point, so it would be easy not to see the USB logo even though it actually does appear.