I am NOT a lawyer and the following is therefore NOT legal advice.
 
A criteria for trademarks in the US is that your trademark applies ONLY
for a particular trade.  The trademark has no value outside the particular
trade where it is being used by a company.
 
As an example, if LSoft has not specifically registered the LISTSERV
trademark for schedule 13 (Firearms, ammunition and projectiles;
explosives;fireworks) and also has not used them in the firearms trade
otherwise, I can legally sell a gun under the trademark LISTSERV and LSoft
can not do anything to stop me.  There are 43 classes listed in the
International schedule of classes of goods and services. The current
fee for registering a trademark in the US is $245 for EACH class.
 
You can NOT register a trademark for something that you do not sell.
Unlike DNS names you can not, in anticipation, hoard trademarks. Also in
the US registrations have to be renewed every 10 years.
 
You do not have to register a trademark for it to be enforceable but
registering a trademark allows you more protection.
 
A real world example is the British vacuum cleaner: VAX.  It does NOT
infringe on Digital's trademark since Digital had not started selling
vacuum cleaners with the name VAX before the British company started
selling vacuum cleaners.
 
Also, if IBM does not sell food, I can legally sell potato chips and call
them IBM chips.
 
 
Eric was note sure he understood "Interstate commerce":
Until the US Civil war in the 1860s, the United States was a plural word
and states had total authority over commerce within their borders.
Interstate commerce is very much like what the European Union is heading
towards.  I strongly suspect that Europe in about twenty years will
resemble the legal organization very similar to where the United States
was about 100 years ago.  Currently European legal structure resembles the
United States of about 150 years ago.