Web Mistress...
Some big ISPs, and even some smaller ones, cache their DNS, meaning, they only update their dns record on a certain periodic rate.
24-72 hours is about how long it take for a dns change or a new domain to "propogate" through the web...
Is why it's REALLY a good idea NOT to let your old ISPs know you're moving your site, until it's been up on the NEW ISP for a couple days...