Mac App Store Stuck - Cannot Sign In

Hello,

I've recently upgraded my Mac OS X from 10.6.2 to 10.6.6 .
Now the App Store is installed and shown in my dock , but I can't use it because it cannot connect to iTunes Store .
When I open the Mac App Store , "Connecting to iTunes Store ..." appears after a few seconds but it really does not do any other actions .
The main problem is that I can't even sign in with my Apple ID . When I click "Sign In" from the "Store" menu , the login form appears . After entering my Apple ID and password , the loading icon appears , and the applications sticks in this situation ( nothing happens after that ) .
Maybe I should notice a few things :

* I'm sure that I have an active Internet Connection and there's no problem with it .
* I'm not using FruitMenu and similar applications ( which makes problems for App Store and are already discussed in similar topics )
* I'm not using any firewall or something similar that prevent app store from any connections .
* I already tried opening iTunes manually ( as described before ) but nothing changed .
* I tried signing in with other Apple IDs and the same error occurred .
* I can use my Apple ID in iTunes Store , Apple Developer Section , ... without any errors .
* I'm sure that I'm not making mistake in entering ID and Password .
* I was monitoring my network activity and I found out that no request or packet is being sent by App Store application through Internet ( after the application sticks in that situations )
* I quitted every other apps while doing this , but nothing changed .
* I'm using iTunes 10.1.2 and App Store 1.0 (54)

Any ideas and helps are really appreciated 😟 ... ( I really need this )
Thank You