Here Come The Tweets: Gnip Offers Access to Full Twitter Archive
allthingsd.com
allthingsd.com
Here's the initial Gnip announcement from almost 2 years ago where Twitter somewhat explain their rationale:
http://allthingsd.com/20101117/gnip-becomes-twitters-first-a...
"The Gnip Historical Power Track product is a job-based system and usage is reliant on a multiple-request API. Generation of data via the Historical Power Track product requires two distinct steps: requesting a job quote/estimate (including projected cost, data volume, and time) and accepting or rejecting a job quote/estimate."
(Disclaimer: I work for Gnip)
http://english.stackexchange.com/questions/45136/difference-...