Owners of three letter domain names do come at risk of being disputed by large corporations using trade marks as part of their arsenal. Current claims through the court give great insight in to the future of domain owners and company owners.
The number of pending and recently settled claims on three character domains is staggering. Here is a portion of the current disputes :
Rcu.net (claim denied)
Ucp.com (transferred, domain may have been stolen)
There have been cases due to bad faith were the domain has been released back onto the domain market so the trade mark holder has to chase the domain. The outcome is extra legal costs and a long parking period for the domain.
Further Information : http://domainnamewire.com/2008/05/29/acronym-domain-name-owners-stave-off-attacks/