New vacuum for 7.2

I'm a little confused about the new vacuum for 7.2. The documentation
says that the new vacuum doesn't lock the tables, but the old version
of vacuum can be invoked by using the 'full' option.

When would I need to use the old version of vacuum? It says that the
old version shrinks the on disk size by removing out-dated tuples.
Does the new version do this eventually?

Thanks.
-Tony