There's 4 different interfaces that use the same OWA URL but function completely differently. There's the web based email that people typically think of when someone says "OWA". There's WebDAV, which uses HTTP or form based authentication (to pick up the proper cookies) and then uses special URLs and WebDAV HTTP verbs to perform actions relative to retrieving/sending mail, listings, calendar, etc (this is in the process of being deprecated I believe). There's Web Services, which is (if I remember correctly) a SOAP API that provides the same functionality as WebDAV (and some additional functionality) which has become the preferred method as of Exchange 2007. There's ActiveSync, which uses WBXML formatted message passing (it's more complicated, but more robust than WebDAV and Web Services). If you're using k9mail to talk to an Exchange server, it uses WebDAV. If you're using the "Work Email" app, it will use ActiveSync, Web Services and WebDAV, depending on which is enabled. If you're using TouchDown, it will use whichever of the three you select.

Frequently in configurations, WebDAV access, Web Services access and ActiveSync are all enabled when OWA is enabled for the Exchange server. It is only in rare conditions when you will see OWA enabled and not one of the three.