virtualbox.org

Is there a tutorial for this, similar to the Windows counterpart (http://forums.virtualbox.org/viewtopic.php?f=2&t=9697)?

Not really. If you check the forums a bit better with Google search, you will find some information on how to do it, but a real howto isn't here. The Windows representation for hard drive devices are \\.\devicepath\harddrive1\ or something like that. You have to check that for the actual path. It's in the properties of the hard drive I think.

But I will give you this: Windows isn't always so fond in giving users raw disk access. From what I know, Vista and especially 7 can give you a hard time.

I don't mind experimenting a bit, but before I do so I need some reassurance:

Assuming that I give virtualbox raw access to one partition of my hard drive (not the entire thing!). If I mess up in whatever way inside the virtual machine, will the rest of hard drive (i.e. the other partitions, MBR, etc.) remain absolutely unchanged? I cannot afford damages to my host OS, Win7 - inside another partition of the same hard drive - but I don't mind messing the partition containing my newly installed ubuntu.

As long as you specify a partition and only have access to that partition, nothing else should be touched. Let's say you install Linux with it's boot loader in the MBR, you won't see it when you boot your system. Linux would not see your Windows installation either, because it can only read the partition you selected for raw access.