I can't seem to find any bugtracker, so I'm reporting this here in the hope that someone from Sun see's it - I guess "Suggestions" is a better place than the "Other guest" forum?
The bug comes from this forum thread: http://forums.virtualbox.org/viewtopic.php?p=45350
Basically on a 64-Bit host CPU+OS, Solaris 10u5 or 10u6 have serious problems running. This is on Vista and Fedora9 host OSes.
The 10u6 install seems to go OK, except for a warning about the default boot device changing in the BIOS at the end of the install. The first boot never completes, you get a "bad PBR" message.
With 10u5 you can get through first boot, but if you install a patch cluster to upgrade the kernel, you end up in the same position as 10u6 - no more boot. If you install Guest Additions you end up with no X11.
I thought it might be a SATA issue, but you're no better off with IDE, you do seem to get different error messages though - which I've since lost, something about being unable to identify hd?
With the VT-X option disabled, or a 32-Bit host you can install Solaris 10u6, kernel patches and Guest Additions, it basically works fine (not tried SATA yet).
Seemless mode goes nuts though - basically takes over the whole screen and then flicks between the menubar and the host OS desktop.
In regular "console window" mode, maximising/minimising the VirtualBox window causes gnome_segv to die.
Fullscreen mode doesn't stretch to fit the screen e.g. 1024x768 guest window doesn't switch to 1280x1024 host viewport.
So Guest Additions still don't really work either, but I guess that's a separate issue.