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.