I have a client who's users use a common account to log into windows, they then access SharePoint 2010 and authenticate via their AD credentials. They access Outlook Web Apps via a page that has an iframe to the OWA site. Is it possible for their AD credentials they used for SharePoint auth to be passed to OWA so they do not have to authenticate twice?
If you are going to use a common account that is different than the user's individual account then you may want to consider setting up something that can support Single Sign On. One example would be setting up the system for Claims with an ADFS provider. The claim can persist and be used to authenticate the users to OWA. A second option would be to use something like Microsoft TMG or UAG which can also provide Single Sign-On capabilities.
Updated with resource links
Configure SharePoint 2010 for ADFS http://blogs.technet.com/b/speschka/archive/2010/07/30/configuring-sharepoint-2010-and-adfs-v2-end-to-end.aspx
It sounds to be me like the Kerberos to be one way to achieve this quickly, but not entirely maybe. So you would need to configure Kerberos for your farm and AD domain. Find examples and walkthrough : http://technet.microsoft.com/en-us/library/gg502599(v=office.14).aspx or http://blog.blksthl.com/2012/09/26/the-final-kerberos-guide-for-sharepoint-technicians/
Furthermore (as Mike also proposes) another mechanism could be ADFS, (to my knowledge OWA is not claims aware therefore it would be impossible to pass-it on further).