There's an interesting but rough new service, FindMeOn, that's tackling a problem a lot of Web users have: too many personal profiles. This service is designed to let you create one personal profile on its site, and then export the data to other sites, such as Facebook, MySpace, and Match.com.
FindMeOn will let you export pieces of your profile to specific sites. You might want to leave out your age or religious affiliation on MySpace, for example, but include such info on a dating site. (What if you want to say different things on different sites? I don't know if FindMeOn will address that.)
The site is in the preview stage right now, and from a usability perspective it's pretty much impenetrable. So I can't recommend you put effort into it yet. But it's a neat idea and I hope the team does the necessary work on it to make it easy to use.
There's another site in development, and not yet open to users: Profilactic (normally I don't comment on product names, but I will here: Yuck). This service will aggregate the content from your various social sites, such as MySpace, Flickr, and Digg, into one place. It sounds to me as if it'll be creating, essentially, a metaprofile page--one that's made up of all your online personalities. If you want to keep your personalities different on different sites, that's not something you're going to want to share. It might be useful to see what people are saying in the comments fields on all your profiles, although many profile sites now have RSS feeds, and simply subscribing to those would do the same thing.
These two services (writing multiple profiles and tracking their activity) really should be available as one product. Anything that can make it easier for users to manage their participation in the increasing number of online communities is a good thing--but we shouldn't need more than one service to do it.