<p>I am already excited about document databases and especially about CouchDB's simplicity. But I have a hard time understanding if such databases are a viable option for multi user systems. Since those systems require some kind of relations between records which document databases do not provide.</p>

<p>Is it completely the wrong tool for such cases? Or some tagging and temporary views are the way to accomplish this? Or else...</p>

<p>UPDATE:<br>
I understand the answers so far. But let me rephrase the question a bit. Lets say I have a load of semi-structured data which is normally a fit for CouchDB. I can tag them like "type=post" and "year=2008". My question is how far can I go with this type of tagging? Say can I create an array field with 10.000 names in it? Or is there a better way of doing this? It is a matter of understanding how to think in this document based sense.</p>

Multi-user systems do not
require
relational databases, though RDBMSs are a staple technology for data storage/retrieval for a vast number of (especially CRUD) applications.
If you want to read-up on document/object -oriented, distributed database solutions of yore, search on "Lotus Notes/Domino" (it's a mature technology/product in this area that's good background knowledge in how applications are designed in a document-based paradigm. Classically, it's really good at workflow type applications).
On CouchDB specifically, check out:
http://wiki.apache.org/couchdb/
(this shouldn't be a surprise)
http://seanoc.wordpress.com/2007/10/12/more-on-couchdb/
(easy reading description overview)
http://twit.tv/floss36
(Podcast interview all about CouchDB)