If you receive a message like this:

Hey I’ve been following you for awhile now and I figured I should let you know about the raffle Tumblr is having for a couple iPhone4s (they made a staff post about it). Enter if you want, all you do is go to the sign up page and put in your email here

It’s a scam.  The link at the end of this message is a shortened link.  Many people use link shortening services for legitimate reasons, but shortened links are a great way to mask URLs that would otherwise mask suspicion.  If you ever encounter a shortened URL you’re not sure about, you can use CheckShortURL to find out where they lead to without visiting the site.  In this case, the URL leads to a site that definitely isn’t Tumblr, and a search of Tumblr doesn’t bring up anything about a raffle except other people talking about raffle messages they’ve received.

The account it was sent from appears to be a legitimate (if rather poor-quality) blog, so it would appear that the scammer may have hijacked the blog in question.