What is the effect on your credit rating immediately after you get a mortgage?

You must have some sort of valid credit rating in order to get the mortgage to begin with (unless you went through one of those "last chance" guys that charge 25% on the principal) and as long as you don't default on any payments your credit rating can only go up. The rating is a reflection of your ability to pay off debt. Getting a mortgage will affect any future amounts you may wish to borrow only in regard as to the amount. You can only carry so much debt based on your income, regardless of how fabulous your credit rating may be.