If you get a "want to win an iphone" DM, don't click the link...it's spam!!! Pls RT!
At the time of writing, two hours after this tweet, 18 people have retweeted, so people believe it can happen, which it can't.
Spam is unsolicited, and one would have to follow the twitterer to receive it. Following someone means that you are opting-in, or soliciting, so by definition, any tweets, direct or otherwise, from the twitterer can't be spam. The best way to not get DMs from spammers, is to simply not follow any spammers. Looking at the originator's follower counts (and that she's in PR), it's not a total shock that she might be following a spammer.
This tweet has two features of a good hoax, in that it sounds like a well-intentioned warning, and it has a request to spread it around. I get plenty of other hoaxes (usually virus / trojan warnings) from friends via email or facebook message, and I usually reply with a link to a hoax-buster site. Simply googling for a key phrase in the message is enough to find one. In this case, as the message is only two hours old, there aren't any results, so I'm going to claim ... FIRST!