I ran into an issue today opening up an ASP.NET page from a SharePoint 2010 sample project from MSDN today:

The Site URL property of the project does not contain a valid URL. To ensure that designers work as expected, set the Site URL property of the project to the URL of the local SharePoint server. Close all designers, and then open them again.

If you are accustomed to working with VSeWSS 1.3 then you may try to set the URL from the Start Browser With option from the debug tab in the property pages for the project, but that will not work. 

The Site URL property to which this error is referring can be found in the properties tool window when you have the project selected.  If you cannot see the properties tool window then you should be able to make it appear by hitting F4 or selected View > Properties Window from the menu.  From there, you just type the URL into the Site URL property and you should be good to go.

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