A high serum lactate does not necessarily mean a bad prognosis: it all depends on the cause.
I made this diagram as a mnemonic for the causes of high lactates:
Additional information added 1st June 2011: One cause of an elevated lactate may be artefactual, secondary to interference with the assay (used on ABG machines) by ethylene glycol. The assay may also be subject to interference from certain drugs at toxic levels such as isoniazid, acetaminophen and thiocyanate. This information is from the Renal Fellow Network.