I've been playing around today with VirtualBox, after finding that the latest version claims to run on Solaris 10.
Which is true, but you need to jump through a few hoops first.
First, you need to make sure that libGL.so can be found. I guess this varies a bit depending on whether you're using mesa or have the nividia drivers, but I ended up setting LD_LIBRARY_PATH_64 to /usr/X11/lib/amd64.
Then you need libXinerama; if you're running an older version of Solaris (my test machine was running S10U3 == 11/06) then applying patch 125726-02 will do the trick.
Then if you're running 64-bit you'll need a copy of libdlpi. I just snarfed a copy off one of my test opensolaris boxes (actually indiana preview 2 - I have seen comments that the one from the official OpenSolaris release won't work as it's too new).
(Yes, I realize there might be a bit of a chicken and egg situation there!)
Then I tried booting the indiana preview 1. Which worked just great. No network, but I expected that. The only glitch I had was the key to escape from the guest - which is set to Right Ctrl by default, which I discovered I don't have. I reset that to some other key that I do have and don't use for anything else.
Having learnt from that, I had a go at the OpenSolaris 2008.05 release having found a CD that I had brought back from CommunityOne, and that worked fine (and picked up the network).