Appraisal Visits

When I get an appraisal, does someone actually come to my house to look at it?
In most circumstances the appraiser does have to come out to your house to view the property.  This is especially true since home values have been dropping which results in a high Loan To Value (LTV).  In previous years, when values where increasing quickly, and the LTV was low enough, an appraiser did not have to go out to the property. 
I'm not an expert, but I think there are lots of different kinds of appraisals.  BPO is a broker's price opinion, where noone comes to the home.  It is used a lot in evaluating a loan to sell on the secondary market.  A full appraisal is when someone will physically come out and go through your come.  With a drive-by appraisal, no one goes inside the home.  And with an AVM, a computer tries to evaluate the value based on some calculations.
And how long is an appraisal actually good for?