(Using VirtualBox with Win7 host/guest...)
Is it the possible to connect to a wireless network in a virtual machine, but keep the host os disconnected?
[useful in places where wi-fi is free]
Yes, depending on your wireless card. If your card fully supports promiscuous mode, then the guest OS should be able to monitor/send packets on that interface with WinPcap or a similar library if supported by VirtualBox without the host OS getting on the network.
However, most wireless cards do not fully support this (either driver or hardware limitation), which will limit or make it impossible to do this.
If the Wifi is free, is there a specific reason you need the host to remain disconnected? It would probably be much easier to have the host connect and then bridge with the guest, but I don't know if you have some reason not to do so other than academic reasons.