I've been working on setting up a VM to play around with SharePoint 2010 and kept running into a weird issue where I would be prompted for my credentials repeatedly and I was denied access to the site entirely, even when using an administrator account.
I had set up my SharePoint web application using a host header; for example, beta.dev.com.
Meanwhile, I was able to access the same site from my host environment without any issue. What gives? Turns out that there is a slight registry tweak you have to make when working with host headers and IIS.
Phil Harding has a good writeup with links to the Microsoft KBs and additional details.