SQL Updating column after adding it giving "Invalid column name" error

I have the following SQL in SQL Server 2005 but I get an error stating "Invalid column name 'ExpIsLocalTime' (ln 7) when I run it:

IF NOT EXISTS(SELECT * FROM sys.columns WHERE Name = N'ExpIsLocalTime' AND Object_ID = Object_ID(N'[dbo].[tbl_SessionsAvailable]'))
BEGIN
    ALTER TABLE dbo.tbl_SessionsAvailable ADD
    ExpIsLocalTime bit NOT NULL CONSTRAINT DF_tbl_SessionsAvailable_ExpIsLocalTime DEFAULT (0)

    UPDATE  tbl_SessionsAvailable
    SET     ExpIsLocalTime = 1
END
GO

This will be in a script file that may be run more than once so I'm trying to make sure the UPDATE only runs once. Is there something about BEGIN/END that delays the execution of the DDL statement?