I disagree with other posters, I think this is a great concept. It would be fantastic for brands to share promotional offers and voucher codes as people would have to tweet to revive access to the code therefore spreading the promo further.
How is this a good thing? You want users to visit a promo page, where they're forced to retweet the content before getting access? This sounds awfully familiar to those sites that ask you to share a referral link with 10 friends, or complete a survey before receiving a free gift that doesn't exist. Also reminds me of the days of browsing warez online. Click these three links to vote for us on Top 100 sites, then you'…
>> "before receiving a free gift that doesn't exist"
Presumably if this was done by someone like Coca-Cola you would receive something for sharing the page. The spam problem could be improved by asking them to tweet their own content + link rather than a predefined message. e.g. Tweet your favourite thing about [brand] and get [reward]. And the tweet would be [your message] + link.
These techniques could be used in interesting ways - most people just don't bother.
>> "You want users to visit a promo page, where they're forced to retweet the content before getting access?"
Also if the user is getting something of value I don't see the problem with asking them to do something to get it. They aren't being forced, they are being given a choice. Do this thing and get rewarded. Don't do it and life goes on.