Introducton
NNNN.org's are great domains. They are short, numeric (which is an advantage in itself) and dot org - which is a very popular extension and one that seems to be associated with trustable web sites. They are usable for almost any purpose.
Other extensions
The fact that NNNN.com, NNNN.net and many other have been registered mean they are already a great investment. NNNN.coms are worth hundreds, and it is likely NNNN.org's will come to be appreciated too. Even NNNNN.com domains have been sold out, and have value which makes buying into NNNN.org an obvious option.
How many have been registered?
As of August 2008, ~90.5%% of these names have been registered. There are just 942 of these names left, or 9.42% (as of 5 October 2008).
Mobile phones
With the rapid growth of the web, especially on other devices such as mobile phones, shorter and easier to type in names are in demand. It's much easier to type in four numbers than it is to type in four letters on a mobile phone, because you may have to push a key down twice/three times/even four times to get the letter you want.
Premiums
All premium number dot orgs have already been sold out. These are domains that are only made up of certain 'premium' numbers, and are generally worth more.