I am slightly unsure of which direction you wish to connect.

If it's PC->N900->3G->Internet, install the "Bluetooth Dial-Up Networking" on your N900, and tell the PC to use this "modem" to dial the Internet. Both the PC and the N900 can be active at the same time.

If it's N900->PC->whatever->Internet, I think that's probably possible but haven't explored it.