MSDN has a great article on Mass-Hosting High-Availability Architectures.  Anyone who has been even close to MH-HA environments knows the complexity (not to mention pain) that comes with living in this space.  However, for those not as familar with this space, this article offers a great overview of many of the ‘ingredients’ that have to be dealt with and why.   It’s not a a cookbook nor is it the ‘silver bullet’ (doesn’t exist), but for those uninitiated, it provides great info.  Pay special attention to the discussion of App Pools.  You’ll thank me later.  Check it out.