My N900 has a SIM card with a flat-rate 3G data plan. My laptop hasn't. What do I do when I want to use the Internet on my laptop somewhere that doesn't have WiFi? Well, there are many options:

Option 1: N900 as a USB modem

Use the provided USB cable to connect the N900 to the laptop. Choose "PC Suite" mode on the N900 when you get the USB connection menu. The laptop now sees your N900 as a bog-standard USB 3G modem. Use Network Manager to connect to the internet.

Pros: no extra setup required. The N900 and the laptop can both access the Internet at the same time. Cons: you have to use a USB cable (I hate cables). You cannot ssh into your N900 (and ssh is my primary file transfer protocol between the laptop and the M900).

Option 2: N900 as a Bluetooth DUN modem

Install Bluetooth DUN support from Maemo Extras. Then use it like you would any other phone that has Bluetooth DUN.

Pros: no cables. Cons: Bluetooth is the worst technology ever. I never had it work reliably. Plus, Network Manager in Ubuntu 10.04 doesn't support Bluetooth DUN (it supports only Bluetooth PAN, as far as I know).

Option 3: N900 as a WiFi access point with Joikuspot

I haven't tried this.

Pros: simple (hopefully), no cables required. Cons: Joikuspot is non-free. I'm not an absolute zealot, but I will avoid closed-source stuff when open-source alternatives are available.

Option 4: N900 as a WiFi access point with Mobilehotspot

I haven't tried this either.

Pros: it's an open-source app available from Maemo Extras. No cables required. Cons: requires a non-standard kernel (or so I've heard). Way outside my comfort level.

Option 5: N900 as a WiFi access point with shell scripts

Here's the shell script I run on my N900: share-wifi. It sets up an ad-hoc WiFi network, and starts a DHCP and DNS server (dnsmasq). Sadly, it cannot set up connection sharing (NAT), so I rely on OpenSSH as a SOCKS5 proxy. The whole setup is like this:

You want the latest firmware (PR 1.2) to avoid this bug. You need to have OpenSSH installed on the N900. Also, setting up key-based authentication makes it more convenient. The script assumes that you've set up sudo on the N900 so that you can run any command as root. You need to have wireless-tools installed. It's in the main SSU repository so you should be able to sudo apt-get install it (if it's not preinstalled; I don't remember). On the N900 run share-wifi in a terminal (optionally passing a WiFi channel number from 1 to 11, in case you need to avoid interference with nearby networks). On the laptop connect to the new n900 WLAN and run ssh -D 1080 user@n900 . You will get a shell session; the SOCKS proxy will be active while it is open. Reconfigure your laptop to use a SOCKS5 proxy on localhost:1080. For GNOME systems I've a couple of shell scripts: proxy-on and proxy-off. For applications that do not use the GNOME proxy settings (such as Subversion access over SSH), use tsocks. When done, hit Ctrl-C on the N900 to terminate the sharing script.