This excerpt taken from the TM 20-F filed Jun 24, 2009.
Marketable securities and Investments in Affiliated Companies
Toyotas accounting policy is to record a write-down of such investments to net realizable value when a decline in fair value below the carrying value is other-than-temporary. In determining if a decline in value is other-than-temporary, Toyota considers the length of time and the extent to which the fair value has been less
than the carrying value, the financial condition and prospects of the company and Toyotas ability and intent to retain its investment in the company for a period of time sufficient to allow for any anticipated recovery in fair value.