Update: An extensive documentation has been published talking about SharePoint 2010 performance at http://technet.microsoft.com/en-us/library/ff758647.aspx
Microsoft has published first performance tests for SharePoint 2010 Beta. Although they are very simple documents is a very good starting point to have an approximate idea about SharePoint 2010 performance. I understand that these documents will be followed by a more extensive document like the Planning and architecture for Office SharePoint Server 2007. In addition you can find all architecture resources about SharePoint in this URL:
- SharePoint 2010: http://technet.microsoft.com/en-us/library/cc261834(office.14).aspx
- SharePoint 2007: http://technet.microsoft.com/en-us/library/cc261834.aspx