Other than hiding Activity Monitor from us in SQL 2008 (it's on the toolbar now) it has some great new features that make the role of a DBA almost obsolete.
Well, not quite, but it does make things much easier.
Install SQL 2008 and you can even run things like Policy Based Management and Activity Monitor (Super 2008 Ed'n) against SQL 2005 & 2000 instances.
My favourite trick is to show query plan on an active query, show missing indexes, then implement. Instant performance improvements... and here's another way to track performance (or just snoop).
The new Activity Monitor has another trick up its sleeve: If you open it and then expand the first band of information just below the four graphs, you'll see a list of processes that you can order, sort and filter. If you right-click any process, you'll see an option to "open in Profiler". Click that, and you'll open Profiler with a default trace right on that SPID. Very useful to quickly identify the actions of a connection.