dnsmap was originally released back in 2006 and was inspired by the fictional story "The Thief No One Saw" by Paul Craig, which can be found in the book "Stealing the Network - How to 0wn the Box".
dnsmap is mainly meant to be used by pentesters during the information gathering/enumeration phase of infrastructure security assessments. During the enumeration stage, the security consultant would typically discover the target company's IP netblocks, domain names, phone numbers, etc ...
Subdomain brute-forcing is another technique that should be used in the enumeration stage, as it's especially useful when other domain enumeration techniques such as zone transfers don't work (I rarely see zone transfers being publicly allowed these days by the way).
If you are interested in researching stealth computer intrusion techniques, I suggest reading this excellent (and fun) chapter which you can find for free on the web.
I'm happy to say that dnsmap was included in Backtrack 2 and 3 - although the version included is now quite dated - and has been reviewed by the community:
