From my experience, if the computer running Management Studio isn't in the same domain (or trusted domain) as the SQL server, you can't use Windows Authentication to connect to the SQL server.  

The trick of mapping a drive to the SQL Server will work for other things that use windows authentication (such as computer management, SQL 2000 Enterprise Manager / Query Analyzer) just won't work with Management Studio.

One technique I haven't tried (and I could see the strange MS logic in using this) is to put an entry for the SQL server in the control panel, user accounts.  If this doesn't work, you will have to use a SQL Account.

Personally, I believe this is a design flaw.  In my environment, I constantly have consultants, vendors, etc. working on specific pieces of equipment.  By that definition, their laptops aren't in my domain and they can't use windows authentication from them.