you could also just have apache::session update that column as well
on the data store by overrideing the store mechanism
If you're using MySQL 4.1 or above for your db store, you can also use a timestamp column like:
ts timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP
MySQL should keep the timestamp current for every update for this column. Of course the earlier comment about the extra work for the db still applies. Leaving off the "on update CURRENT_TIMESTAMP" part and overriding the update method would save you the extra update per update. But if your db is pretty unloaded, this is certainly the easiest way to go.