While VirtualBox is available as a downloadable OpenSolaris package from the download page at virtualbox.org, I find it much more convenient to use the Package Manager GUI or pkg on the command line to install and update packages.
Sun provides a VirtualBox IPS package (and some others like Flash Player) from a separate "extras" repository. However, you need to obtain a key and SSL certificate before you can access this repository, which are available for free from https://pkg.sun.com/register/ after logging in with your Sun Online Account.
Once you obtained and installed these files in /var/pkg/ssl (detailed instructions are provided on the download page), you can add this repository as another "authority" and start looking at what packages are provided:
$ pfexec pkg set-authority \
-k /var/pkg/ssl/OpenSolaris_extras.key.pem \
-c /var/pkg/ssl/OpenSolaris_extras.certificate.pem \
-O https://pkg.sun.com/opensolaris/extra extra
$ pkg list -a 'pkg://extra/*'
NAME (AUTHORITY) VERSION STATE UFIX
SUNWadmj (extra) 0.5.11-0.101 known ----
SUNWjsnmp (extra) 0.5.11-0.101 known ----
SUNWwbapi (extra) 0.5.11-0.101 known ----
SUNWwbcou (extra) 0.5.11-0.101 known ----
SUNWwbdev (extra) 0.5.11-0.101 known ----
virtualbox (extra) 2.0.6-0.101 known ----
virtualbox/kernel (extra) 2.0.6-0.101 known ----
web/firefox/plugin/flash (extra) 9.0.125-0.101 known ----
So there is not that much to download by now - some additional Java packages and the Flash plugin for Firefox. There is no package for VirtualBox 2.1.0 yet, but I hope this will be updated soon...
