<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<PropertyBag Url="_catalogs/masterpage/default.master" ParentType="File" RootWebOnly="FALSE" HyperlinkBaseUrl="http://sp2010/Test" xmlns="http://schemas.microsoft.com/sharepoint/">
<Property Name="vti_charset" Value="utf-8" />
<Property Name="vti_candeleteversion" Value="true" />
<Property Name="vti_modifiedby" Value="SHAREPOINT\system" />
<Property Name="vti_cachedneedsrewrite" Value="false" />
<Property Name="vti_level" Value="1" />
<Property Name="vti_rtag" Value="rt:7CDE768C-FCE1-46E8-9165-FDA564419D9A@00000000002" />
<Property Name="vti_cachedbodystyle" Value="<body scroll="yes" onload="BLOCKED SCRIPTif (typeof(_spBodyOnLoadWrapper) != 'undefined') _spBodyOnLoadWrapper();">" />
<Property Name="vti_title" Value="<asp:ContentPlaceHolder id="PlaceHolderPageTitle" runat="server"/>" />
<Property Name="UIVersion" Value="3" />
<Property Name="vti_parserversion" Value="14.0.0.4536" />
<Property Name="vti_cachedhastheme" Value="false" />
<Property Name="vti_foldersubfolderitemcount" Value="0" />
<Property Name="vti_hasdefaultcontent" Value="true" />
<Property Name="vti_setuppathversion" Value="4" />
<Property Name="vti_metainfoversion" Value="2" />
<Property Name="vti_canmaybeedit" Value="true" />
<Property Name="vti_generator" Value="Microsoft SharePoint" />
<Property Name="vti_etag" Value=""{7CDE768C-FCE1-46E8-9165-FDA564419D9A},2"" />
<Property Name="vti_cachedtitle" Value="<asp:ContentPlaceHolder id="PlaceHolderPageTitle" runat="server"/>" />
<Property Name="vti_contentversion" Value="0" />
<Property Name="vti_progid" Value="SharePoint.WebPartPage.Document" />
<Property Name="vti_timelastmodified" Value="2010-02-10T16:12:34Z" />
<Property Name="vti_sourcecontrolversion" Value="V1.0" />
<Property Name="vti_timecreated" Value="2010-02-10T16:12:34Z" />
<Property Name="vti_folderitemcount" Value="0" />
<Property Name="vti_docstoretype" Value="0" />
<Property Name="ContentTypeId" Value="0x01010500CE60EDAC92E8C942BFB82AF8AFC4A5F4" />
<Property Name="vti_filesize" Value="29039" />
<Property Name="vti_sourcecontrolcookie" Value="fp_internal" />
<Property Name="vti_author" Value="SHAREPOINT\system" />
<Property Name="vti_setuppath" Value="global\default.master" />
<Property Name="vti_metatags" Value="GENERATOR Microsoft\ SharePoint progid SharePoint.WebPartPage.Document HTTP-EQUIV=Content-Type text/html;\ charset=utf-8 HTTP-EQUIV=Expires 0" />
<Property Name="vti_replid" Value="rid:{7CDE768C-FCE1-46E8-9165-FDA564419D9A}" />
<Property Name="vti_docstoreversion" Value="2" />
<Property Name="vti_cachedcustomprops" Value="vti_title" />
</PropertyBag>
</Elements>
As you can see the relative URL of the file is specified along with a separate attribute indicating the base URL it came from. You also need to specify something for ParentType. For now I know that you can specify File or Folder there (although I am guessing it might take Item or something like that).
Now for the most part all of these properties listed above are internal properties that you would never care about setting, but as you guessed you can specify your own properties as well. The export site as template functionality does its best to export everything it can and that includes internal properties as well. This is pretty interesting feature and I think it could remove the need to write a lot of code in my future. @SPKyle might still want to write code to store properties on list items, but I will take a pass. :)