The simplest solution is to buy a blackberry charger and blackberry usb cable.

Seriously, try it out for yourself with a friends blackberry charger/usb to test that it fits perfectly.

This is because the problem is not with the actual port on the n900, but with the size of the nokia micro usb on the charger/cable, this same problem was persistant with my nokia n96.

Blackberry has been more accurate with the sizing of the micro usb on their charger/usb cable, so if you buy those, you will never have a problem with the cable being lose when plugged into the n900, it will fit perfectly and charge properly, without every breaking or causing any sort of damage to the phone.

I have personally tested this with both my nokia n96 and n900, and currently use blackberry chargers and blackberry usb cables for my n900 rather than the nokia ones!