MSN havent updated all of their servers to the new cert version, so this was causing intermittent problems. The fix here is updated (if you dont want to or cannot upgrade to latest version yet) and the below information is correct and working as of 24/11/2010.
So your Pidgin chat under Linux or Ubuntu is giving SSL ‘Unable to validate certificate error’ when connecting to MSN’s omega.contacts.msn.com server? Don’t worry you are not alone, this is happening to a lot of people (see here). Here I’m going to tell you how to fix it so you can get chatting again.
A lot of advice is flowing around to update to the latest pidgin to fix the problem, many people are reporting upgrading does not resolve the issue, so these steps will manually insert the new certificate.
Step 2 – Install
Place those files in /usr/share/purple/ca-certs (or /usr/local/share/purple/ca-certs depending on your *nix distribution (note that you may need to remove the ‘_2010′ from the files if you find the problem remains)
Step 3 – Use
That should be all you need to do to log into MSN! Give it a go
Thanks to Mohammed for the updated info. You can find the full info from Pidgin here