I have exactly the same problem with the N800 that I used to have with the N700. Getting the stupid thing to stay in USB mode is a case of complete luck.

Powered off N800. Connect the USB cable and power on holding down the power button. The USB logo appears in the corner. Release the power button and it vanishes causing the device mount to fail.

Does anyone else experience this and how do I get around it? It makes me dread update time.

*Edit* Aghhh. I can't believe how stupid I've been. *hits self over the head with the home key*

Ignore me and I'll go quietly kill myself in a corner