It appears that passport.net's SSL certificate has expired and nobody at Microsoft's replaced it (perhaps somebody should drop them an email?). Try connecting to, say, https://nexus.passport.com and you should get a warning about an invalid (because of expiration) SSL certificate.

The official MSN client logs on regardless of whether the SSL certificate provided is valid or not (crappy), but I've disabled this in my client.

Anyways, I thought I'd let people know what's up in case they were suddenly experiencing troubles logging on to MSN Messenger via MSNP8 with secure clients.