I have configured and running NAT port forwarding in version 2.0.4. (XP SP3 Host, Ubuntu Intrepid Guest).
After upgrade to 2.1.0 NAT port forwarding not working.
Downgrading to 2.0.4 and it works again.
VBoxManage getextradata intrepid enumerate
VirtualBox Command Line Management Interface Version 2.0.4
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Key: GUI/SaveMountedAtRuntime, Value: yes
Key: GUI/LastWindowPostion, Value: 102,35,800,644
Key: GUI/Fullscreen, Value: off
Key: GUI/Seamless, Value: off
Key: GUI/AutoresizeGuest, Value: on
Key: VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/HostPort, Value: 2222
Key: VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/GuestPort, Value: 22
Key: VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/Protocol, Value: TCP