Looks like the Lync 2013 App for mobile client is now released and in the Windows Marketplace. Was very happy when I saw it this morning until I tried to sign-in. It doesnt seem to support Office365 Accounts yet as I am recieving the error.
You can’t sign in with this version of Lync. Please install Lync 2010
A bit gutted to say the lease but hopefully this issue should be sorted by Microsoft soon. Lync 2013 Mobile client will only work with Lync 2013 Server with CU1.