Also possible, but not covered in this guide:
Plug your iPhone into the computer and load up iTunes.
Click on the Info tab and check Sync contacts with and make sure Windows Contacts is selected. Click apply.

A warning may pop up asking to replace all your stuff. Click no if you get this dialog box:

Select Merge Info on the next dialog box (if it appears):

It will now start importing your iPhone contacts:

It should finish within a few seconds:

Open Windows Live Mail via Start Menu->Windows Live->Windows Live Mail:

Click cancel if you are prompted to create a new account. Dismiss any other dialog boxes to get to the main window.
In the bottom left click Contacts:

In the top right click the menu button and select Export and then Business card (.VCF):

Create a new folder called contactfolder (for example) or choose an existing one to dump the .VCF files in (there will be one VCF file per contact):

Click OK and it will export the .VCF files:

Close all applications on the N900 that are using internal storage.
Connect the N900 to the computer with the USB cable and select Mass Storage Mode on the N900. It will then show up and you can open its folder:

Drag the folder with the .VCF files onto the N900:

Safely remove the N900 and then disconnect the USB cable.
On the N900, open X Terminal. We will assume that the folder with the .VCF files is named contactfolder. Note: to get the > symbol, press the Sym key on the N900.
cd MyDocs cd contactfolder cat *.vcf > allcontacts.vcfThis will merge all of the .VCF files into one big one named allcontacts.vcf. Close X Terminal.
Open the Contacts application on the N900. Open the menu (at the top of the screen) and select Get Contacts.
Next select Import contacts file and navigate to Nokia N900 and then contactfolder.
Then tap allcontacts and your contacts will be imported.
That's it. I hope this has helped! :)