I've seen a lot of issues with using the MSXML DOM when trying to get credentials from the owaauth.dll for FBA authentication. The managed classes seem to work all the time and in all versions.
However, the MSXML DOM wont always reuturn cookies and sometimes just plain fails in the calls. Each version of MSXML has an http method at the version of the MSXML component and versions of that method which are for each prior verion of MSXML. You will find that you will get varrying mixed results per each of these methods, per version and per service pack of MSXML. Because of this, you cannot be sure that the same code using MSXML will work on another box if it is used to go against the owaauth.dll Also, note that you really should be using the managed classes under .NET and not the XMLDOM.
On a side note, if you are going against Exchange 2007, you really need to think hard of why you are using FBA authentication. People have been using FBA authentication for WebDAV against Exchange 2000 and 2003 for one reason - to work around the issue of FBA being turned on for OWA, which casuses WebDAV to only be able to authentication using FBA (unless one of two specific work-arounds are used). With Exchange 2007, the virual folder and its settings are split-up and seperate for OWA and WebDAV so that one will not affect the other. This means that WebDAV can authenticate using Basic Authentication instead of FBA even if its turned-on for OWA.
How WebDAV - Use Basic Authentication with WebDAV even when FBA is enabled.