I think it will work out of the box. If i connect a symbian device in PC Suite Mode to my Mac it does exactly the same like the N900 in this Video: http://www.youtube.com/watch?v=5vSqGSG9riU
Starting with Jaunty the Network Manager of Ubuntu also supports wireless broadband. I use this with my N97 and it is working perfectly. I think it will also show up as serial connected modem in Ubuntu and Network Manager will recognize it.