I'm throwing this post out there that brings together the things I learned as I worked to get the free Windows 7 Public Beta running on Ubuntu 8.10 Intrepid Ibex via VirtualBox. Hoping this post helps those trying to do the same, in all my Google queries on these issues information was sparse... a few hours wasted on my part will hopefully save you time. This is written in a "just the facts" style and not a full "how to" but should get you where you are going avoiding some of the pot holes.
Proof it's possible!
Second tip here... no matter how much you want to use Linux/FireFox/wget to download, I tried them all... Don't! Follow their rules, fire up a copy of Internet Explorer however you can to download the ISOs and then copy them over a network or large flash drive back to your Ubuntu machine. The ActiveX (I know, gross...) downloader appears to have less issues corrupting files which just plain saves you time.
Getting VirtualBox ReadyEither... You already have VirtualBox running on Ubuntu and have run a number of virtual OSs without any issues... Great!... Nope, thats what I thought... Make sure you force an upgrade to the very latest VirtualBox available, doing anything on older version causes blank screens, "Aborting" of the OS and other things that waste your time. Or... You are new to VirtualBox, in this instance you are probably better off, go to VirtualBox.org and grab the latest version.
Setup a new VirtualBox "machine" choose Windows Vista as the type and give it at least 1GB of RAM and 20GB of hard drive. In the network area I suggest you choose the "Intel PRO/1000 MT Desktop (82540EM)" adapter rather than the default, Windows 7 seems to like it better from scratch. Connect your downloaded ISO to the CDROM and it should boot from it by default. You won't even have to ever even burn that DVD, beautiful.
Installation Bumps and BruisesInstall speed, blank screens, file expanding... Installation will be SLOW don't shut it down and restart when you are impatient, I did that about 5 times, it is working unless the VirtualBox aborts of the installer throws errors.
Can't find "install.wim" - Your ISO is corrupt, you are going to have to download again unfortunately please make sure you download if from Microsoft directly and via Internet Explorer and the custom downloader. I downloaded two corrupt version before I gave in and followed the rules too, thats 5+ GB of wasted time and downloaded bytes.
Virtual machine crashes and returns to Virtual Box with the status "Aborted" - For me, all these issues were due to running and old version of VirtualBox and corrupted ISOs.
I hope some of those issues I dealt with help you get you copy up an running, now go beat the tar out of it and let Microsoft know if you will be using it in the future or sticking on the Linux side of the fence. -Ben