Hi,
I started to work with VirtualBox few months before and since then I had a problem with networking.
I bridged mode I had no internet connectivity and there was no way to see the host (even the host was able to ping the guest!).
I discovered the source of the problem and decided to tell it here to help others may have this behavior...
The source is my Cisco VPN Client (I checked version 4.5, 4.6, 4.8 and 5.2).
This client has a 'Stateful firewall' on the options menu. If this firewall is on it blocks the communication between VirtualBox guest and host...
I hope this helps
Peter