I was doing some research on DNS the other day, specifically on caching and TTL values. My current client has a DR site that needs to have functional names (e.g., www.domain.com, mail.domain.com) as quickly as possible. We were first under the impression that it would take 48-72 hours for DNS changes to propogate. I did some research and found some good articles on the topic:

• Wikipedia article on DNS
  
• Why DNS Based Global Server Load Balancing (GSLB) Doesn’t Work
  

I was also trying to query the DNS server for the TTL values, but was coming up short with the built-in nslookup. So I found a great little port of dig for Windows which works great. It also has some good info on DNS, caching, and TTL.