garbage collection & use for delete

[akcom]

As I currently understand it, "deleting" an object simply marks it as 
ready for deletion when the garbage collector makes its period sweeps. 
After skimming through the phobos std source, I saw very VERY little use 
of the delete keyword, especially in regards to classes.  My question 
is, how does the gc know when those classes are no longer being 
referenced? Also, are the explicit delete's I'm using on .dup'ed 
strings, classes, etc useless?