Which web technology to learn for an experienced C++ developer?
There are many options.

Since you already know (and is learning about) Java, one option is to use GWT for both server and client. This can help you in that you do not need to learn another language (JS/HTML/Python/PHP etc). If your portal is going to be big, using Java can help you organise the application better - usually JS/HTML based applications are not very suitable for proper organisation, even if you use good JS Libraries like jQuery or YUI. Having a good organisation can help a lot - during updation and modification later. 
If your planned venture is a single/two person venture or if it is time bound - where time to market is everything - then I would not suggest the earlier approach - especially if your server side part is expected to be big. 
Java is a slow language to write code in. A project which you will take say 6 months to write in Python will take you close to 1 year + in Java. In such a scneario, I would prefer Python - it is a proper language - unlike PHP, and you create code with good organisation there too - albeit a little less organised than using Java. 
Please note that if your client side code is much more complex than your server side code, then going with GWT will do you no harm. But if your server side code is very complex compared to the client side, then I would suggest Python.
Another point is to use existing Web Frameworks to ease your work. For Python, Django is an excellent choice. This itself will decrease your work time by 50% or more, while making your code much more secure and scalable.
Friends, 
I've got some exp in c++ and now kind of starting my way to J2EE (to survive:))). Meanwhile, I've got a plan to venture in to a web portal my own. But with very little experience in web technology, I'd need to start from scratch. I'm little confused on which way to go and I'm here. 
PHP, Python or JSP, considering the fact that, anyway I've got to learn J2EE at my work.
Would that be worth to learn PHP or Python to develop a portal which I expect to get 80-100K 
hits per day "IF" everything goes well OR jsp would be sufficient? 
Many thanks