A KEY FEATURE to the G1 Android handset is that it syncs itself with your Google account – especially for the handsets contacts/addressbook. Don't spend half a day trying to import from Nokia's PC Suite. It won't work. Use the SIM importer facility instead.
There's plenty of information available on how to import addressbook entries into Google's Gmail from an Apple Ithingey or Mac. But the reality is that many potential G1 phone users will be migrating from a Nokia.
Managing a Nokia's addressbook can be done using the Nokia PC Suite software. Plus PC Suite can sync itself with Microsoft Outlook's addressbook. So you can have all your mobile phone numbers and email addresses in one place.
A word of advice. Don't do this because you could end up like the INQ with 1199 contacts in your Nokia's handset. (The handset reported that there was still plenty of room for more).
The phone becomes unusable since with 1199 entries it takes ages to find the telephone number you want. Restoring to the original phonebook produced a much more manageable 349 contacts.
The conversion process should have been simple. Just select all the address entries in Nokia PC Suite; save the results as a .csv (database) file and import the file into Gmail.
It works but there's obviously a clash in the way Nokia stores information in Vcard (vcf) format. All the phone numbers went into the Gmail addressbook in the 'notes' field. Not the 'mobile phone' field. Disaster.
The INQ considered using a Vcard editor such as Vcard Organizer v1.2 by Stefano Toniolo but it isn't freeware as advertised. You can make changes with a standard text editor like Notepad, anyway.
So, after hours of messing about, the INQ suddenly discovered that the G1 can import numbers from the SIM card. The snag here is that you're usually limited in the number the card will take. The latest SIMs take 200 numbers while older SIMs were only 100.
The INQ was also going to attempt to import email addresses from Hotmail into Gmail. The very latest version of Hotmail swears it can export to a csv file but when the INQ tried it, the system reported a network error.
There's a long-winded way and details are provided here but here but by this time the INQ had lost heart.
Incidentally, for Cupertino fan boys the way to export from an Ithingey is here. Or for Mac bods try here.
Finally, to answer an INQ reader's question – it is actually possible to get the inbuilt Microsoft Messenger client working in Europe. But you have to switch to Wi-fi mode.
That's probably because – as the handset indicates – it can't find you telephone number to charge you for the messages. µ
No easy way to do this. I also am going from a Symbian Nokia to G1 and I spent about a day re-entering the contacts since there is little point in having all of the info in the "notes" field - useless.

If anyone finds a shortcut, please let us know. Too late for me, as I already wasted hours on this, but some other poor G1 soul may benefit.
I spent a couple days trying to get my phonebook from my Motorola Z6c onto my G1. I finally got it moved over correctly and posted the solution here

http://groups.google.com/group/Gmail-Help-Chats-and-Contacts-en/msg/53f9787ee7a93da2

And again: Google, you suck. Your online help sucks. You should be totally ashamed.
Hi, 

although I never tried what you did, I noticed one step in the middle of you article. The ".csv" one. This should be "Comma-Separated-Values" text file, which should be easily imported into Excel (if there are less then 65536 records :) 
Don't know whether the file have fixed order of columns or the columns are identified by header (=value in first row). In both cases content of "Note" column can be moved into "Phone No." column either by swapping of columns or just swapping of headers respectively. Then, of course, one have to export the list into .csv format (use different filename, please :) and try his/her luck with importing.
Export the non-standard CSV from the G1 into a spreadsheet application as columns, then compare/re-jig that to the CSV file from Nokia. Work out the best way to rename the fields then save as CSV and re-import back into the G1. Just echoing what was mentioned in the article but it sounds pretty quick to do.
This is a complete pain in the neck! If it were just a matter of adjusting the CSV file output by Nokia PC Suite a bit using Excel, there wouldn't be much problem, but it isn't that straightforward... In the case of S40 phones like the 6230, you can have MULTIPLE general phone numbers, mobile phone numbers or notes entries per contact. PC Suite only exports the FIRST one of each of these if you save as CSV. It does output all of them if you save as a vCard (.vcf) but then how on Earth do you get that into the G1 or gmail? The answer is that you can't: at least not with ALL the fields correct and intact. Haven't found any solution so far, despite trying four or five different conversion apps and much Googling. Android NEEDS proper vCard support. This G1 is going back to T-Mobile if I can't get it sorted out...