Are all your profiles up to date? Are you tired of filling out your profile on websites and online applications? Wouldn't it be useful to just maintain one or two central profiles that would automatically update all the others?The promise of "portable profile" is just that. You can use one profile across the web, and sign into sites without having to fill out all the usual registration stuff.Of course, this raises major privacy concerns. Each site that you use your portable profile on will have access to your full profile information, as well as possibly knowing what other sites you're registered with.As a website owner, there are benefits to allowing users to register with their portable profile:1. You can personalize their experience by knowing more about them,2. There's a  lower cost of password and account management, while drawing new web traffic.3. It lowers user frustration by letting users have control of their login.There are a number of companies offering portable profile, internet ID, and single sign on services already such as Verisign, OpenID, and my favourite

However, Facebook Connect seem to be leading the way with regards to the promise of taking your network of friends and connections around the web with you. There are, of-course, concerns about whether we should trust a commercial enterprises with our precious identity. Nevertheless, I'll highlight Facebook Connect as the principle is important.

From the Wikipedia page on Facebook Features:
Facebook Connect is a single sign-on service that competes with OpenID. The service enables Facebook users to login to affiliated sites using their Facebook account and share information from such sites with their Facebook friends. 
The price of Personalization is Privacy. I don't mind sharing my information if it's going to get me more relevant information and personalized service, but I would expect to be able to control what different people (and sites) can see on my profile. Another concern with single-sign on is the danger of someone getting hold of your one password that opens the doors to your life online. I guess these are the risks we all have to live with. I'd like to get Dominic White's perspective on this.