DB Mirror with SQL Server 2008 Principal and Mirror, SQL Server 2005 as the witness

From reading books online I would say yes.

It looks as though you can run a Principal and Mirror using SQL 2008, whilst using a Witness server on SQL Server 2005 or above.

Taken from SQL Server 2008 Books Online (October 2009):

Software and Hardware Recommendations

We strongly recommend that the witness reside on a separate computer from the partners. Database mirroring partners are supported only by SQL Server 2005 Standard and later versions and by SQL Server 2005 Enterprise Edition and later versions. Witnesses, in contrast, are also supported by SQL Server 2005 Workgroup and later versions and by SQL Server 2005 Express Edition and later versions. A witness can run on any reliable computer system that supports any of these editions of SQL Server. However, we recommend that every server instance that is used as a witness correspond to the minimum configuration that is required for the SQL Server Standard version that you are running. For more information about these requirements, see Hardware and Software Requirements for Installing SQL Server 2008.

For full reference details see: Database Mirroring Witness