Personalization of blocks is part of the Sparrowhawk user interface add-on update that will be available Q1 this year. You can already personalize content in an XHTMLString property that's used inside a content area.

Thanks for your quick respons. Do you have some description of how it will work i Sparrowhawk?

Will it be possible to personalize the items of a single ContentArea property? I'm thinking of a solution where the editor adds visitor groups to  the ContentArea and then drop different blocks in them.
Or are you working on some other way of personalizing blocks?

You will have the option to personalize each item in a content area. You will also have the existing support of personalizing content inside an xhtmlstring property inside referenced block/page. We will release another beta of the add-on, probably this week, with release notes containing more information.

Thanks a lot!

Have a look at this post for now:
http://www.david-tec.com/2013/01/Bringing-back-the-Composer-personalisation-container-in-EPiServer-7/

Yes, I saw that post yesterday. Thanks.