@bdukes@bdukes is right on the money for determining if the schema exists, but the statement above won't work in SQL Server 2005. CREATE SCHEMA <name> needs to run in its own batch. A work around is to execute the CREATE SCHEMA statement in an exec.
Here is what I used in my build scripts:
IF NOT EXISTS (SELECT 1 FROM sys.schemas WHERE name = '<name>')
BEGIN
-- The schema must be run in its own batch!
EXEC( 'CREATE SCHEMA <name>' );
END