Joiku has release beta version of JoikuSpot for linux and maemo platform. By using JoikuSpot you can turn your N900 to WiFi hotspot and can share your 3G connection or Gprs connection with other device like iPod, Laptop or other mobile phone.
I tried to install this on my N900. Initially I had some problem downloading software to my device but after installation it worked awesome. Configuration is quite easy, you can start using it just after installation with default option. Or if you prefer to configure security , implement NAT option or choose IP range , it allows you to change those parameter as well.
If you want to try it out download it from here.
Here are snaps of JoikuSpot working on my N900.