Garbage collection?

[titan.ksc.nasa.gov!mcroberts@ames.arc.nasa.gov]

I've been looking at two different Ada textbooks which have
very different ideas about garbage collection.  The older
book says that a good Ada compiler should take care of it for
you and you should avoid unchecked_deallocation.  The newer
one says that you need to manage storage yourself using
unchecked_deallocation.  I'm curious as to the state of the
art in automated garbage collection.  I'm used to using 
Common Lisp on Symbolics or Unix platforms, and they use a
combination of temporal, incremental and full GCs.  I'm also
familiar with languages such as C++ where you have to manage
garbage collection yourself, but you can implement reference
counts and other sorts of schemes to simplify it.  Is this
something one needs to worry about when using solid production
quality Ada compilers, and is there any negative performance
impact if you don't clean up your own garbage.  In Lisp it is
usually better to let the system do it since it is highly
optimized and done mostly during page faults.