Note: this post is only relevant for people running the SharePoint 2010 Technology Preview.
When I create a new site based on the Publishing Portal template you get a .../Pages/Default.aspx page with an error on it. The error seems to be generated by a ContentByQuery web part (the only web part) on the page. Add ?contents=1 to the url (…/Pages/Default.aspx?contents=1):
Check Out the page, remove the web part (Delete, not Close), and your page starts working again.
Disclaimer: All information in this blog post is based on my personal interpretation of information collected at the SharePoint Conference 2009 and experiences with SharePoint 2010 Technical Preview version provided to my company in the PEP program.