I just downloaded and installed Solaris Express 11 on VirtualBox on my Mac and I was pleasantly surprised by the integration work done during these last two years. Everything I tested is working out of the box, including screen resizing, a traditional paint point of Solaris + VirtualBox.
Beside paving the way toward Oracle Solaris 11 and being one of the first Oracle branded release of Solaris, there are many new functions for developers and system administrators.
I was particularly impressed by the extremely simple installation process, that started with OpenSolaris and is now live for all current and future Solaris users. You can have a preview with this video.
You can also download a prebuilt Virtual Box VM, ready to use.
Give it a try !