The official decision probably hasn't been made. They might have had a couple interviews scheduled after you, and that is probably adding into the delay. It really doesn't take long to get a letter out once the final decision has been made. Some companies just sit on their ass until the last minute, so I probably wouldn't worry too much if they told you they liked you a lot.
Also did the HR people hint at an offer or did your interviewer?
From my experience:
Internship 1: interview Thursday, verbal offer Thursday, official emailed offer Monday.
Internship 2: interview, verbal offer 5 weeks later, official offer from HR 1 hour later.