A recent article in TechHive points out that you can now use line breaks in tweets. However, this is nothing new; I've been doing this for a long time, with my Twitter clients. Just press Option-Return to make a line break that does not act as press Return (or sending a tweet). It's worth noting that Option-Return works in just about any text field, including those where Return sends text (such as in Messages). This isn't a new hint; this feature has been around for ages, but it's good to know.
It's worth noting that my Twitter client of choice, Twitterrific, already handles Returns as line breaks; you have to press Command-Return to send a tweet.
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