The Lync Mobile client for Windows Phone 7 is now available at Microsoft marketplace for download.
Clients for iOS, Nokia and Android has been submitted on their respectively marketplace for aproval, so hopeful these users will get there client soon.
Also the push notification service is not working at the moment for WP7 – it seems like Microsoft needs to enable something on the push.lync.com domain. This means that you won’t get any IM’s when Lync Mobile is not open. Hopefully they get it fixed soon.
UPDATE: the push notification is working now.