When I got my n900, I subscribed to a second account with new phone number for it and kept the old phone number with my old (very small) phone.

One reason was that the 2nd account with 3.5 Mbps flat rate data transfer was cheaper than just 1 Mbps flat rate data added to the old account.

Additional benefit is that I do not need to carry n900 everywhere. I mostly use it as a computer (like mini laptop).