When surfing on one of the SO sites (SO, SF or Meta, and soon SU), and accessing a user's profile, it would be easier to have links to his/her profile on the other sites.
- give us a better view of his profile ("developer only?, IT guy? advanced computer user?")
- increase the visibility of the other SO sites ("wait a minute, there is a ServerFault site? or a Meta site? Interesting, I will check them out")
- allow the user to quickly switch between his/her different profiles and check the evolution of the rep or comments on the other site.
Those links could be located on:
- the profile page,
- but also on the "recent" page.
(in the latter case, they would be visible only by the user, allowing for a quick check of latest responses/comments between the sites)