Not so long ago, I wanted to make an API which would take a blob of text and return sentiment prediction on it. But I couldn't get the data for the same. Anyone knows any good source of sentiment data? (Apart from the usual product review data from Amazon, etc.)
If you're interested in learning about sentiment and the machine learning behind it, two of the early 'seminal' papers (somewhat outdated by current techniques, but really understandable) are Turney 2002 (http://acl.ldc.upenn.edu/acl2002/MAIN/pdfs/Main425.pdf) and Pang and Lee 2002 (http://portal.acm.org/citation.cfm?id=1118693.1118704). Wiebe's work is also worth a look (she's quoted in the OP).
Good places to start, at least; ping me if you want some more.