Cannot connect to any online resource

Try adding the following to devenv.exe.config (in Common7\IDE folder):

<configuration>
  <system.net>
    <settings>
      <servicePointManager expect100Continue="false" />
    </settings>
  </system.net>
</configuration>

This is a known issue with some proxy servers that will be addressed in a future release.

The following worked for me, I got this from the blog entry listed below.

in the devenv.exe.config file, in the <system.net> section add:

<defaultProxy useDefaultCredentials="true" enabled="true">
    <proxy usesystemdefault="True" />
</defaultProxy>

reference: http://gurustop.net/blog/2010/08/10/visual-studio-2010-extension-manager-online-gallery-behind-internet-proxy/

I had to do a bit of all the above to get mine working. My system.net settings:-

<system.net>
    <defaultProxy useDefaultCredentials="true" enabled="true">
        <proxy proxyaddress="http://your.proxyserver.ip:port"/>
    </defaultProxy>
    <settings>
        <ipv6 enabled="true"/>
        <servicePointManager expect100Continue="false" />
    </settings>
</system.net>