This morning pidgin was complaining about my MSN accounts and the cert coming from omega.contacts.msn.com. I seems that the cert was changed.
The fix:
Using Firefox visit: https://omega.contacts.msn.com/
Right-click on the page (an error page) and select page-info
Select the Security tab and click "View Certificate"
In the Details tab click export and save it in the certs dir in your .purple folder
ex:(Win XP) C:\Documents and Settings\{username}\Application Data\.purple\certificates\x509\tls_peers
(if you can's find this on your machine a simple search for tls_peers should point you in the right dir)
In Pidgin disable then enable any MSN accounts and it should connect.
For people who don't want to grab the cert themselves a helpful blogger has a link to a copy of the cert (and similar instructions) here: http://blog.andreineculau.com/2010/11/pidgin-and-msn-certificate-error-for-omega-contacts-msn-com/ BUT BE WARNED that certs are the sort of things you should get from the site itself if you care about security.
Hope This Helps,
Brent