Normally, in the scenario above the 2nd connection would wait for a metadata lock on t1 until con1 finishes the transaction, or lock_wait_timeout occurs, whichever happens first. However, if server is running as a Galera node, ALTER TABLE is executed right away, despite the locks that con1 holds; and the transaction in con1 will be rejected.
Explicit BEGIN/COMMIT here are just to make the test deterministic, in fact it happens just as well on a single-statement transaction with autocommit.
Limitations page (http://www.codership.com/wiki/doku.php?id=limitations) mentions LOCK statements, but does not say that internal locking is also not supported. Besides, this kind of scenario does not involve any other nodes.