I hope that this site  a secure site running disk encryption. You know what
happens when you post confidential data on a web server.

If you've already addressed the security  needs of data like that, here's a
way to create the column  itself.

weblogs.asp.net/.../practical-tweaks-creating\
-a-social-security-number-column.aspx

To display it in hyphenated form, you'd have to either (a) create a custom
field, or (b) create your own page that splits the output appropriately.

How about setting up three columns for each section of the SSN and then
concatenating them using a calculated column? You could still apply the same
numeric validation to each of the columns as well. Popping all of this into an
Infopath form would make it loot a little better, too.

I tried Mark's suggestion using custom site  columns for the three sections
and it worked as described, including the validation. Having three separate
entry fields is a little ackward in practice though.

I poked around and found this article:

Office Space: Custom Field Types for SharePoint 2007 - Ted Pattison

msdn.microsoft.com/en-us/magazine/dd727508.aspx

I haven't tried this yet, but it appears to be exactly what is needed,
including sample code.