I just installed VirtualBox 4.1Beta3 and the corresponding extension pack.
PXE-Boot is not working in this version, I got the following error message:
Unknown Ethernet Controller! Aborting PXE boot...
PXE-M0F: Exiting Intel PXE ROM.
Fatal: Could not read from the boot medium! System halted.
Nothing changed since VirtualBox 4.0.10 where PXE-Boot worked just fine.
The virtual network adapter is "Intel Pro/1000 T Server".