Doing my experiments ....
Until now i put n900 in a stable state of powering the vbus few seconds.
according standard there is a time frame for connection 4.9 s . In this state ( a_vbus_vld ) n900 drive the vbus to 2.7 V ( 0.8< vbus_otg < 4.0 V ) and driver expect a_wait_bcon event. I could not imagine why bcon don't happen, leading switching the bus to reset state, instead i got unhandled disconnect interrupt
I found some limitations of devices offending last usb 20 standard and a lot of variants of usb 20 standard itself and a lot of obscure notes regarding functional implementations.
I think to write some code in a PIC microcontroller to inject a fake B_device connection signal into N900 but it is a little bit hard to switch to a real usb device hub or memstick fooling them they are already connected.