If you are running into problems using Apple Remote Desktop (for example, you can't see anything but a black screen when you try to control a remote host), and have checked for UDP connectivity over the oft-cited port 3283, try this as well...
Check to see if the intervening routers and firewalls are allowing fragmented packets. Disallowing fragments is a serious show-stopper for ARD.
Hope this helps someone who may be frustrated by the problem...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030328171852761