Merb was never a fork of Rails. It was a separate project, initially created to address a particular shortcoming of Rails (file uploads, IIRC), and grew into a competing implementation that solved a lot of the same problems that Rails did in a simpler, more elegant way. The Rails core team realized that Merb did a lot of things better and absorbed most of that goodness.
SlimGems isn't Merb. It's not new, it's not doing anything better. The only thing it has going for it is "Hey, it's not being run by these same assholes." Well, that doesn't help me much. SlimGems isn't going to ship with Ruby 1.9.3, and it doesn't solve any problems that RubyGems doesn't solve. Loren could be making a better RubyGems, but instead he's making an older RubyGems.
Well, the other thing it has going for it is that it works. So far I've had no problems with SlimGems on the half dozen or so projects I've tried it with. The same cannot be said for RubyGems 1.8 and most of the releases since 1.3.7.
SlimGems isn't Merb. It's not new, it's not doing anything better. The only thing it has going for it is "Hey, it's not being run by these same assholes." Well, that doesn't help me much. SlimGems isn't going to ship with Ruby 1.9.3, and it doesn't solve any problems that RubyGems doesn't solve. Loren could be making a better RubyGems, but instead he's making an older RubyGems.