That is exactly what we do at my place of employment. We keep a text file with a list of tables that have changed. Our script gets the tables from source control (CVS) and then parses the table file (i.e. Table1.TAB) for column names. If the column name doesn't exist on the destination, an ATLER TABLE command is executed to add the missing columns.

Our updates are done directly to the dB. If you do it directly to the dB like you want or you choose to output to a SQL script like mpeppler suggested, you'll probably want to make sure that either way the script(s) are re-runnable.

It is not often that we DROP or modify columns. For me, this has to be done another way. Our DBMS repication can't handle column DROPs or UPDATEs without intervention.