tedster - 3:11 am on Jan 3, 2011 (gmt 0)
You can get a rough idea of the discussion volume by use a Google search for [site:twitter.com keyword]. Google partners with Twitter and has access to all the Tweets - but the volume is so immense that they do not tap the whole thing. It's still better that Twitter's own search tool right now.
The challenge with that Google research - and with all third party tools as well - is extracting or analyzing the sentiment. Did the poster actually "want" the item, or did they criticize it, or reject it, or already have it, or just wonder about it? It's not as cut and dried as a search volume number - not by a long way.
Extracting sentiment from text is a vast challenge and most current "listening solutions" can at best be called early beta. Not only that, but the best ones are quite pricey and mostly to be used only by very large enterprises.
If you have that kind of budget, then the place I'd suggest you look for options is Forrester Research. Their "Wave" report follows the industry and they create quarterly reports about the third party listening platform services that are available. Forrester Wave: Listening Platforms Q3 2010 [blogs.forrester.com]