You can only use 'default.master' or 'custom.master' as strings. If you download the Windows SharePoint Services 3.0: Software Development Kit (SDK) from here: http://www.microsoft.com/downloads/d...displaylang=en
and navigate to section: "Customizing Master Pages in Windows SharePoint Services", this will state the following:
Referencing the Master Page from a Content Page
Windows SharePoint Services provides several tokens to reference the master page—two dynamic tokens ("~masterurl/default.master" and "~masterurl/custom.master") and two static tokens ("~site/default.master" and "~sitecollection/default.master").
Hope this works for you. Suggest you use the token 'custom.master' to avoid an unsupported scenario. You may also upload the master pages to the MOSS gallery as per http://msdn2.microsoft.com/en-us/library/aa830818.aspx
The SDK could have been a bit clearer.