I have similar issues with USB, except that sometimes it works, sometimes it doesn't. Unplugging it (on the N900-side) and plugging it in again usually helps.

Funny I never have this when I use the adapter that came with it so that it can be charged with my old N810-charger.

If it helps anybody: When The device doesn't react to the USB-connection, here's what I see on my PC:

usb 7-2: new full speed USB device using uhci_hcd and address 2
usb 7-2: device descriptor read/64, error -71
usb 7-2: device descriptor read/64, error -71
usb 7-2: new full speed USB device using uhci_hcd and address 3
usb 7-2: device descriptor read/64, error -71
usb 7-2: device descriptor read/64, error -71
usb 7-2: new full speed USB device using uhci_hcd and address 4
usb 7-2: device not accepting address 4, error -71
hub 7-0:1.0: unable to enumerate USB device on port 2