Excellent information. Minor point, according to Microsoft's documentation on sp_getapplock the error trap code should actually be as follows:

IF @Result < 0

BEGIN

ROLLBACK TRAN

RAISERROR('Procedure Already Running - Concurrent execution is not supported.',16,9)

RETURN(1)

END

sp_getapplock will return 0 if it locks immediately and 1 if it locks after waiting for other locks to clear. All failures will be negative values.

http://technet.microsoft.com/en-us/library/aa933410(v=sql.80).aspx