I had this issue before. I don't remember how I solved it. Many others have the same issue, but couldn't find a legit solution.
Some guy in this forum: http://code.google.com/p/android/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars&groupby=&sort=&id=41231 suggests to add a line to the android.bat file. I did what he said. I added " -Djava.net.preferIPv4Stack=true" but no luck, I cannot open my SDK manager with that line!
I have also did the https vs https thing, which I read elsewhere; no luck.
I added som manual site like "-Djava.net.preferIPv4Stack=true", no luck. How come the SDK Manager should be problematic in this basic level?! I cannot install a package.
java is allowed in my firewall.
I'm not using proxy and tried two different wifi connections.