A renderer plugin will need to be made for this functionality.

Heres how i would do it, if you want to restrict some text to registered members, do something like the following;
[registered_users_only]the text[/registered_users_only]

And then have the plugin find and replace that appropriatly, and if the user is not registered then replace it with a chosen message from the plugins settings saying something like "<em>There is content here that is only avaliable to -registered- users</em>" with a link to the register page.

Alternatively the plugin could also have
[guests_only], [members_only]

There may be a inbuilt solution but this is my best guess.

This is a brilliant idea, but also assumes I know how to write plugins and such *grins*. I am, admittedly, a total newb to b2evolution and blogging in general. Have yourself a peek when you have a moment.

http://www.studioreflections.com/blog

Is there any particular reason why users should register if not to gain access to something in particular? I've been asked a few times what they get if they register, and in all honesty, I don't know the answer to that.

:-/