One problem related to air bag has been reported for the 2008 BMW Z4. The most recently reported issues are listed below.
Light indicating passenger airbag was disabled began coming on while passenger in the seat. Within a few weeks of noticing this, passenger airbag always indicates off. Many 08 models were recalled for passenger airbag mat failures, but not the Z4 (too few cars made?) I called BMW-USA and a case agent called me back. BMW would take no responsibility for the failure.