A long article appeared in Technology Research News : http://www.trnmag.com/Stories/2004/111704/Software_sorts_out_subjectivity_111704.html
and a shorter version at MIT's Technology Review magazine http://www.technologyreview.com/articles/04/11/rnb_111604.asp?p=1

An excerpt from the longer article:

Longer-term, they are aiming to develop methods that can handle variations in language, said Lee. "This is very important in dealing with on-line text, since Internet sources can very widely in form, tenor and even rammaticality," she said. "One can get reviews from the highly-edited New York Times or from a stream-of-consciousness
personal Web log."

The ultimate aim is to be able to handle rhetorical devices like irony and sarcasm, said Lee. "Given that even humans are occasionally misled by such rhetorical devices, this is going to be very challenging," she said.

People are incredibly creative at expressing negative opinions, said Lee. For example, this sentence not only contains no obviously negative words, but has a lot of potentially positive words: "If you think this laptop is a great deal, I've got a nice bridge you might be interested in."