I succeed to install DSM 4.1 into VirtualBox, but I wasn't able to install DSM 4.2 into VirtualBox... until I found that a .IMG file could be converted into a .VDI file.
The command line to convert is :
VBoxManage convertdd DS3612xs_3202-Repack.img DS3612xs_3202-Repack.vdi
So now I'm happy to have a DSM 4.2 in VirtualBox.
Thanks guys for all your work ! It's so cool...