I hope this is not too confusing:
My setup is a laptop with Ubuntu on it, and now i'm running a Virtualbox with Windows XP in it. Is it possible to tether my iPhone (3gs) directly from the virtualized Windows XP?
You need to enable USB pass through on Virtual Box (If tethering via cable) and/or use a USB wifi card and pass that through to virtual box if tethering via wireless. (I am guessing you want it to be completely isolated from the Ubuntu host).
Then, you need to tether the iphone like normal to the machine - jailbroken or otherwise, I will leave that up to you!