Many places will interview you anyway, just so they can justify to themselves that they really were thorough.

My first job out of college, they made me an offer at the beginning of the interview process, because I'd interned with them before and they had a fairly good idea what I could do. The rest of the interviews were basically get-to-know-the-people-hired-after-your-internship.

Also, at bigger companies, your referral may not be from the group or specialty you'll actually be working with, and so you'll have to go through the interview process itself to make sure they can stand you. My Google application process was like this, and my IBM one would've been had I pursued it. It's still a big positive, that someone in the company is willing to vouch for you.

The interview process, or some sort of skills assessment process, is still necessary even after the referral. Otherwise, you run the risk of hiring someone akin to the boss' nephew who claims web programming skills, but has never been in the same room as someone who has those skills. Relying on friends and colleagues for advice on who to hire is good, but it can turn bad if that is the only data point you rely on.