yeah. but a couple of days ago I tried exactly this (using laptop internet connection on N900) but via either USB or Bluetooth with no sucess :-(

that's right. it happened to me too i've been trying this on my brother's 5800 since long time ago. and i tried a lot with the n900 and i had to go into process, bridge connection, change IPs.... many stuff to make it work

Please help! I can't connect my n900 with Vista in ad hoc mode.

I moved to win7 but i will try to help you.
1- ofc your laptop/pc should have wireless wifi capabilities and it should be connected to the internet via ethernet not a wireless connection
2- Go Network and Sharing Center in your Control Panel
3- Choose to create a new connection then choose to ad-hoc
4- Security type should be set to Shared, Encryption type to WEP
5- to turn on internet sharing through this network you should set the internet connection (not the "local area connection") to shared.
6- To do that click on your internet connection then on properties. Then go to sharing tab and select the "Allow other network users to connect through this computer's internet connection". The home networking connection should be set to "Wireless Network Connection"
7-Now you have to change the IP address of the wireless adapter to match that of the ICS. Click on "Change Adapter Settings" in Network and Sharing center, then right click on wireless network connection -->properties--> double click on Internet Protocol Version 4-->Click on "Use the following IP address"
8- In IP address enter :192.168.1.1. Then press Tab, the subnet mask should become 255.255.255.0 if it doesn't enter this value in it. The DNS server value can be left empty.
9- Now disconnect and disable all you connection then enable them and connect to the ad-hoc you created through the N900.
10- if it doesn't work restart PC. If not try to connect to the ad-hoc then to your internet, or try different combinations.
11- Make sure that if you want to connect to another wireless connection through your pc, you have to set it back to obtain IP address automatically again.

regarding to item number 6.
I don't know why but i can't see "home networking connection" select list in "Internet Connection Sharing" tab