Pre-installed ubuntu with Win7 license / image for VirtualBox
Every developer I know which are using linux or mac os x, still have Windows VM: If you develop for web, you need to test on IE.
Personally, I would never buy an ubuntu pre-loaded because I would need to buy a windows license separately anyway - and this is costly. I would rather buy a notebook with OEM Win7, install ubuntu on it and run Win7 inside VirtualBox.
Providing a proper licese or VirtualBox image for Win7 would be the only option I would buy ubuntu pre-loaded.