How can I query the SQL Server Compatibility Level into a variable?
You could query the system table sys.databases:

-- for SQL 2005+
declare @level tinyint;
select @level = [compatibility_level]
from sys.databases
where name = 'AdventureWorks'

Select @level
GO

-- for SQL 2000+
declare @level tinyint;
select @level = [cmptlevel]
from sysdatabases
where name = 'AdventureWorks'

Select @level
GO