Two things. First of all, VirtualBox definitely supports ChromiumOS's networking. The failure to login is actually Google's PAM service failing to connect or authenticate. (Not sure which). If you watch the network traffic, the VM is actually sending traffic just fine. So, everyone who had failures, just try again.
If all you want to do is test it out and see what it looks like, as others mentioned, log in via chronos, with no password. This is the default shared user account, and will let you at least poke around. This always works, regardless of PAM authentication.
In response to Mooneypilot: Yea, it's pretty much a souped up browser; with a minimal OS, so a user is almost instantaneously dropped into the browser. There is some behind the scenes magic that handles some cool security stuff, but other than that...
Also, if you didn't login with the chronos account, and instead used your Google account info, you wouldn't have to login to Gmail after that, you would be automatically logged in. (PS: I realize that you don't want to do that, I'm just telling you so you can save the step and keep your password close to your heart and fingers.)