I just wanted to try and sum up the different possibilities to to this and make sure I got it right:
a)either you get all posts/tags/categories into the main blog
b)get all posts/tags/categories into another blog, be it a separate wp blog or a subblog of the wpmu install
if you opt for a) you could either use
- an aggregator getting everything into the main blog
- donncha's sitewide tagging plugin
- a combination of leogermani's plugins, letting people vote whic harticles get into the main blog, or manually chose then via his repost plugin
this enables you to show off a cool tag cloud, that uses most of the tags/categories used on your wpmu install without a click on any of the tags in the tag cloud leading away from the main blog
if you opt for b) you can use the same tools as above but your tag cloud will lead away from teh main blog if people click on tags, right?
I mean I have seen the given examples of wordpress.com/tags and wordpress.com/tags/politics but we can't achieve that easily, can we?
###edit###
just noticed edublogs.org/ frontpage. The tags they use there, don't lead away from the main page but still produce a list of posts from the whole wpmu install, any ideas how that has been done?