If the domain market goes nuts.
There were twice as many new domains sold in 2005 as in 2004. The trend is that there will be twice as many in 2006 as there were in '05. That is from an article posted by VURG deeper down in this section. If that growth rate - or anything like it - is sustained for a few more years we will see premium prices for a lot of things.
The trick is to get to that promised land of premium values for our domains without being eatten first by the bad wolf "registration fees".
Say an average 4 character domain goes for $50 in 4 years. How much do you pay in reg fees to get there?
LLLL.com and the top extension (net, org, info, and...) 3 character names will go extinct long before CCCC.coms. I think they will grow in value more quickly.
After you remove the LLLL and NNNN names there are still around a million CCCC names, twice as many as there are possible LLLL.coms. Each level we reach is much larger than the one before. There are almost 12 million possible LLLLL.com (5 letters) domains. (Compare 456,976 possible LLLL.coms)