This is primarily a trademark issue.
<edit>typo</edit>
[edited by: Jenstar at 1:01 am (utc) on June 22, 2003]
The key with cases lately seems to be if you register the "confusingly similar" domain name in bad faith - if you are registering it in hopes that people forget to type the S on the end and find your site instead, the other site would have a case.
Another thing to consider is if you think they would bother sending a C&D (cease and desist) to you over the domain name, and how deep their pockets are.
Yet another option is to register it, redirect it to your site until they complain, then hand the name over ;)