$ virtualbox --help Oracle VM VirtualBox Manager 4.0.4_OSE ..
with kernel 2.6.38-8-generic-pae #42-Ubuntu SMP Mon Apr 11 05:17:09 UTC 2011 i686
that is Ubuntu Natty fresh installation. Adding bridge-utils and relaunching guest solved the problem immediately.
$ virtualbox --help
Oracle VM VirtualBox Manager 4.0.4_OSE
..
with kernel 8-generic- pae #42-Ubuntu SMP Mon Apr 11 05:17:09 UTC 2011 i686
2.6.38-
that is Ubuntu Natty fresh installation. Adding bridge-utils and relaunching guest solved the problem immediately.