[UPDATE, 2011-02-22 09:9 : Last evening I received an email from WordPress.com ‘happiness engineer’ Matthew, who writes:
Hi there,
I’m sorry you are experiencing this.
Domain mapping can sometimes take up to 72 hours from the activation time to fully propagate.
You can change your primary domain name back to josschuurmans.com and you shouldn’t have any
problems.If you have any more questions, feel free to reply back and we’ll be happy to assist.
Best regards,
Matthew
Hm. OK, let’s see, if all hiccups are over now, I just might consider switching the primary domain back to http://josschuurmans.com.
In fact, I’ll do that right away. Thank you Matthew!]
[UPDATE, 2011-02-21 22:18 : Around noon today I left a support request to WordPress.com support, explaining the shakiness of my domain mapping. I really hope someone will get back to me with an answer…
Around 6 pm I got fed up with the situation as I saw someone trying to reach my blog and they couldn’t. So I decided to switch the primary domain to https://josschuurmans.wordpress.com instead of http://josschuurmans.com. This is not how I want it, but I hope that people can now at least reach my blog.
The situation was particularly frustrating since I was getting some incoming traffic from a link in a tweet by Jay Rosen. I have no idea which percentage of those referrals where “Oops!”-ed. 😦 ]
[UPDATE, 2011-02-21 11:11 : It’s been almost 36 hours now and I’m still getting “Oops!”-es. Not good. May have to contact WordPress.com support.]
[UPDATE, 2011-02-20 01:03 : Just when I thought everything was fine, Firefox is giving me new “server not found” headaches… Well, it seems to be off and on. I guess the DNS updates are still a bit sketchy.]
[UPDATE, 2011-02-20 00:45 : After some scary moments, when none of the domain names seemed to get resolves, it seems to work now:
https://josschuurmans.wordpress.com redirects to http://josschuurmans.com
http://www.josschuurmans.com redirects to http://josschuurmans.com
http://josschuurmans.com works as-is
A bit funny though that WordPress.com’s domain mapping doesn’t support primary domains starting with “www.” Or did I misunderstand something?
And I’m praying that the DNS records for my <…@josschuurmans.com> email accounts are still working as they should.
My “old”, now un-domain-mapped TypePad blog will likely die a slow and painful death at http://ojamies.typepad.com/josschuurmans/]
“(…) Warning: when you change the name servers, any custom DNS records such as A, CNAME, or MX records will stop working. Specifically, this can mean that any email addresses you have on this domain (such as mail@example.com) may stop working once the mapping is complete. To continue using them, you’ll need to enter new Custom DNS records once you have set up your domain on WordPress.com. We recommend you prepare the relevant information in advance, before you start the domain mapping. Alternatively, you can map to a subdomain, which doesn’t require changing the name servers and will not effect any custom DNS records. (…)
Schuurmudgeon 
Thank you.. for sharing it. I was bout to buy a domain mapping… The only thing i was confused about is, If I will change Name Servers to wordpress.com then how I will be able to manage my mails coming to that domain. anyways…. thanks for the above information.
I really appreciate that the time and effort that you put on mapping the domain on wordpress.com, you have finally shared it.
and yea.. plz. do reply to my comment, if you come across any solution with this. (mapping domain on wordpress.com with www and still using your mail server in conjuction).
Thanks Again. Good Job Done. 🙂
Ishan, when I mapped my domain to WordPress.com, my email – which runs on a different provider – was not interrupted. So, no problems there.