I suppose if you are really anal, you could offload a copy of all your apps to a folder on your computer. Then you could have a backup of every version of every app you had on your Droid.
you can turn off update notifications by going to the market app, press the menu key and select downloads, then press the menu key and select notifications, then select do not notify me.
i agree with making backups of your apps before you update. in many cases the developers are adding functionality or fixing bugs in the app. it's not always a bad thing to update. if you have backups then it's a no lose situation.