The problem with all these PHP scripts is that they all think they're the center of the universe. To get the functionality I need, I'm already trying to stitch scripts together: PunBB for forum, Wordpress for blog, some script I bought for event/RSVP, etc.
Templating and language systems make combining scripts really hard, beyond the obvious issues like integrating registration. PunBB is now the center of my site. I managed to get PunBB's registration working with some of my other scripts.
I don't want to turn my site into a full-fledged social networking site. The social networking scripts I've tried were all very inflexible. I want to keep my profile pages extremely basic. PunBB already has all I need for a basic profile page, it's just organized in a non-standard, overly complicated way with unnecessary elements like the obsolete IM list.
So I'm not asking for expansion of PunBB's profile pages! My question was about widgets or ways to let users put RSS feeds on their profile page.
The wave of the near future will probably be portable profiles (and blogs) that you can "plug in" to other sites. See also Profilebuilder.