Hey, it's been a loooooong time since I was on here, I'm really glad to see DonationCoder is still around!
I often have clients who change their hosting services for web or email, and a lot hinges around knowing when their domain has propagated over to the new IP address. I envision a utility that I can put one or more hostnames in, and flag their resolved IPs as the "old" hosting server, and set an email address to notify when the IP changes (ie. the domain is now pointing to the new host). You could set an interval on which to perform the domain check, eg. every 1, 5, 10, 60 minutes, 6 hours, daily or however would work best.
If such a tool already exists, I've never encountered it before.