Let's see if you've followed the correct sequence:

1) Pair up the devices (bnep0 will be created)
2) Start the Internet connection on N900 (gprs0 will be created)
3) Start the script on N900 (the forwarding, routing and DHCP established)
4) Press "Acces Internet" (It's call "Tethering" in English version, the Android will attempt to request N900 for IP address and DNS by DHCP request)

The problem usually arises in step 4, because it'd timeout before your android getting the DHCP reply. If this happen, try running the script again. The script is reentranable such that it can be run as many time as you like.

Oh btw, can you show me the screenshot after you've done step 4? It should have some signal.