This excerpt taken from the SO 10-K filed Feb 27, 2007.
Taxes Other than Income Taxes
Taxes other than income taxes increased $2.3 million (17.4 percent) in 2006. This was primarily due to incremental ad valorem taxes on new assets. In 2005 and 2004, taxes other than income taxes increased $2.0 million and $4.6 million, respectively. Increases in taxes other than income taxes have followed additions to plant in service since 2002. Plant in service additions have come through completed construction activities or acquisitions.