Yep, Hotmail does indeed use the Spamhaus SBL as part of their spam filtering and blocking system. I received a report yesterday of this specific SMTP error response received: "550 OU-001: Mail rejected by Hotmail for policy reasons. If you are not an email/network admin please contact your Email/Internet Service Provider for help. For more information about this block and to request removal please go to: http://www.spamhaus.org."
Hotmail has posted more information about the different types of blocks a sender can receive over on their Sender Troubleshooting page.
