I disagree. Twitter is pushing upwards of 3000 tweets per second [1], it'd be nearly impossible to scrape at that rate and go without notice, regardless of how many IPs you've got at your disposal.
Twitter would notice a major scraping operation, but if it's done correctly they wouldn't be able to distinguish between user IPs and bot IPs.
edit: Barracuda already did more than 10% of users just for a white paper: http://www.barracudanetworks.com/ns/news_and_events/index.ph...
150,000,000 registered users only takes 170 days at 10 users a second for a first pass. Focus on frequent tweeters for subsequent scrapes. Even among the ~20% of twitter accounts that are active, most don't need to be scraped daily, and the most active accounts are likely spam.
translates to "Scrape every user" unless you know of some magical way to get list of "active" users.
Guess howmany active users there are? Guess how many servers you need running to get through those in 1 hour. My guess is something much more than $360,000 worth / yr.
look for users who tweeted in last X days? also look for their repliers+buddies, since they too are likely to be active. doesn't seem hugely complicated to me?
Requires you to check aka scrape every user to see which ones tweeted in last X days.
Let's do some math, these are all based on numbers from this past June which have likely only gone up since then [1]:
65 million tweets per day / 20 tweets per page = 3.25 million page views per day
[1] http://techcrunch.com/2010/06/08/twitter-190-million-users/