Ran into an issue today where a clients SBS 2008 server stopped servicing requests made to hostname/owa. I am still unsure as to what caused this issue but find a fairly simple way to recreate the OWA virtual directory for IIS. Many admins are familiar with this procedure / howto for Exchange 2003 but are unable to find something similar for Exchange 2007. While the step that I will describe below is not as in depth as the previous link, it did resolve my issue.
If your OWA is giving you the dreaded 404 treatment try this: Open IIS Manager and delete the OWA virtual directory. Next you will need to open the Exchange Management Shell. Enter in the following command and press enter afterwards:
New-OWAVirtualDirectory -OwaVersion:Exchange2007 -Name "OWA" .
That will rebuild / recreate the OWA virtual directory and start accepting client requests immediately. Please note that this recreates the OWA virtual directory under Default Web Site, not under SBS Web Applications like a default SBS / Exchange 2K7 install.
Hopefully, this one liner will save you some trouble.