I've opened a bug: http://www.virtualbox.org/ticket/6559
Also, my current workaround is to setup a continuous ping to the virtual machine. It seems to keep the network connection alive.
I'm having the same problem.
I have VirtualBox 3.2.8 installed on Mac OS X Snow Leopard Server 10.6.4, Mac Mini server edition. I installed a guest OS, Windows XP SP3, that runs over VirtualBox. I use bridged network to get Windows connected to my router. The problem is, after some time period, say 10 minutes, sometimes more, with no network traffic into or out of the guest OS, the guest OS (which is Windows in my case) becomes un-reachable by other machines (in the same LAN). It also lost connectability to outside world, (Browser can not open any web page, etc.) despite that network properties in control panel keeps telling me it is connected.
I did some tests by running other guest OS VDI file, with same problem. So I guess the problem is Mac OS X Server specific (instead of Mac OS X non-server version)? I've running VirtualBox guest OSes on regular Mac OS X with no such problems...!
Is there any solution to this? I hope it can be solved, it's too annoying!