True. Usually when creating a shell loop like this, I prepend 'echo' in front of 'ssh...' as a dry run first. I also wouldn't actually use something like this for commands that are destructive in nature. Usually it's for things like running informational commands ('rpm -q', 'uptime', etc). Of course anything can become destructive if you fat-finger it hard enough.
Now combine that with dsh (http://www.netfort.gr.jp/~dancer/software/dsh.html.en) and authorized_keys, you can _really_ get in trouble. dsh is similar to this Cluster SSH, but is appropriate for use in scripts, cron jobs, etc.
Example:
dsh -Mg all-machines -F 20 'do something'
-M: Prepend any line of output with the machine that emitted it
-g: runs the group all-machines (just a text file in etc/dsh/group)
-F 20: forks at most 20 copies of ssh in the background so you don't overload the server you're running this from
Like how SSH agent forwarding can allow you to hop from box to box with the SSH keys loaded onto your local machine's SSH agent? Found that one a few months ago, made me reflect on what I've been doing in life the past 10 years not thoroughly reading UNIX man pages.
ssh is magic. checkout -X for X11 forwarding to run remote GUI apps, and -R which lets you set up a reverse SSH tunnel when you need to connect to a machine behind a firewall.
Good point about not needing semicolons for multiple-line stuff (assuming the EOL's are not escaped, as you have it in your example). I blame the PHP I spent all week immersed in...bleh.