Let's say I (100 followers) tweet X. Users A (500 followers), B (300 followers, including 100 that overlap with A) and C (1000 followers, of which 800 are unique) retweet it.
Is there a way to get a report of the total "exposure" of the tweet – all the people who have seen it? In this case, 100 + 500 + 200 + 800 = 1600.
Yes, it could either be that particular tweet is not part of a conversation (i.e No one replied to it or the tweet itself isn't a reply) or it maybe part of a conversation but the other person has protected their tweets.
EDIT
Protecting your Tweets
To protect your tweets Log in to Twitter and navigate to your Settings. Then scroll down and tick the check box Protect my tweets
Then click Save to confirm your changes.
The answers so far will only give you at most 100 retweets. There is an API method that returns up to 100 entries of a cursored collection (so you can page through it to get more), though.
There's more information on the Twitter Developers site at GET statuses/retweeters/ids, though you'll also want to read their page about Using cursors to navigate collections.
Edited to add: That said, the first thread on the Developers site that surfaced in a quick google has @episod, a Twitter employee saying:
You can't likely get to all of them. The data is spread out. Your best bet is to use the REST API to determine many of the users who performed the retweet, but the data will still be constrained.
It's easiest to track retweets as they happen rather than try to find them from the past. Use the Streaming API for that.
Best Answer
You can have a look at TweetReach which looks like it provides the analytics you are after.