virtual machines crash with bridged adapters

Whenever I run a virtual machine with a 'bridged' network adapter, my host machine freezes after a few seconds. The kernel ring buffer shows 'soft lockup' messages.

This did work before and might have been introduced with a recent kernel or virtualbox version.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: virtualbox 4.1.12-dfsg-2ubuntu0.1
ProcVersionSignature: Ubuntu 3.2.0-30.48-generic 3.2.27
Uname: Linux 3.2.0-30-generic x86_64
ApportVersion: 2.0.1-0ubuntu12
Architecture: amd64
Date: Mon Sep 10 10:23:15 2012
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: virtualbox
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.virtualbox: 2012-07-13T10:44:30.068408