Can a virus on a virtualbox windows virtual machine affect my host linux system?

I've always been led to believe that your dual-booted Linux would be bullet-proof against Windows malware even without using VM... that the VM is only necessary to protect Windows.

I'm not aware of any virus that specifically tries to exploit a virtual machine. In fact there is a disincentive to do so because white-hat security researchers set up virtual honey-pots (inviting targets) to examine malware in the wild to catch new samples and examine how they work, and so how to protect against them. Black-hats don't want this to be done, so they try to target only real hardware.

If a virus were to infect your virtual machine, you now possibly have a back-door onto your local network and are therefore more vulnerable to other exploits that could then be remotely initiated. Setup your virtual machine so that it starts with a fresh virgin read-only copy at the start of each session.

P.S. I'm writing this on a Ubuntu 10.04 system and I use virtualbox.

That setup is extremely safe

bugs written for windows code won't run on linux

linux is extremely safe against bugs already

you would have to use your password to allow a bug to install on linux

that is a very safe setup you have, virtually bugproof