This was something that I just found out about a few weeks ago, so I thought I'd pass it along. You know how Exchange 2000 introduced Recipient Policies as a method of controlling which e-mail addresses get assigned to users. Furthermore, there is always a Default Recipient Policy, which is created when the first Exchange server is installed. Addresses can be added to the Default policy, or new recipient policies can be created. As the priority of the default policy is always "Lowest", any new policies that get created will always get applied before the default policy does as they have a higher priority.
Noteworthy here is that I mentioned that you can add e-mail addresses to the default policy. Well, what if you want to add an e-mail address to the default policy AND make it the default reply-to address? If you do this, until you either restart the System Attendant service, or simply reboot the Exchange server(s), access to Public Folders via OWA will not work. I'd suspect that it has something to do with the IFS drive path. You can observe this easily with Exchange 2000, since it displays the IFS drive as the M: drive. You can still view this in Exchange 2003, but you have to do so from IIS Manager. When you look at the M: drive, you'll note that it references an smtp domain. This is taken from the Default recipient policy. If you modify the primary address in the default policy (say you add a different domain), this gets applied to the IFS drive, but the change isn't reflected immediately. The result is that OWA access to Public Folders doesn't work until one of the two above steps is completed. Once you do that, you'll notice that the IFS location changes.
This is one of the reasons that the Default policy should *never* be deleted, and it's also a pretty good reason for implementing additional e-mail addresses (and especially changing the primary address) via a second recipient policy instead of via the default.