Apple has released iTunes 10.5.1 for immediate download through Apple's site with iTunes Match available. It's not available via Software Update yet, and we will have the full changelog up shortly.
iTunes Match is now available as a feature in iTunes and users can subscribe for US$24.99. It's listed as a beta feature, and beta subscribers are warned that their iCloud libraries will be deleted at the end of the beta. To thank beta testers for their participation, Apple is offering them an additional three months on their subscription.
iTunes Match was announced alongside iOS 5 this summer and originally scheduled to debut at the end of October. Apple missed that goal, but the service is finally available as of today.