I'm using VirtualBox 2.2.2, and I've tried bridged networking on a XP host, with Internet access and DHCP-assigned IP addresses for virtual guests. I don't work behind a router. This may or may not work on Vista for you.
1. Install Microsoft Loopback Adapter, and install the VirtualBox Bridged Networking Driver for that device.
2. Configure your "real" physical connection to allow Internet Connection Sharing, and select the loopback adapter you installed earlier.
3. Configure your firewall. It might block DHCP, DNS and other traffic from your virtual guests.
You're all set. Configure the guest virtual settings to use Bridged networking, and choose the loopback adapter. The guest should use DHCP to get its IP address automatically. On my XP box, ICS provides a DHCP server, and all addresses are *only* on 192.168.0.*.
PS. If I'm not wrong, this is what the user manual describes as Host-only networking. When I do use that adapter, it doesn't work.