Data Modelling Advice for Blog Tagging system on Google App Engine
Thanks to both of you for your suggestions.  I've implemented (first iteration) as follows.  Not sure if it's the best approach, but it's working.
Class A = Articles.  Has a StringListProperty which can be queried on it's list elements
Class B = Tags.  One entity per tag, also keeps a running count of the total number of articles using each tag.
Data modifications to A are accompanied by maintenance work on B.  Thinking that counts being pre-computed is a good approach in a read-heavy environment.
Am wondering if anyone might provide some conceptual advice on an efficient way to build a data model to accomplish the simple system described below.  Am somewhat new to thinking in a non-relational manner and want to try avoiding any obvious pitfalls.  It's my understanding that a basic principal is that "storage is cheap, don't worry about data duplication" as you might in a normalized RDBMS.    
What I'd like to model is:
A blog article which can be given 0-n tags. Many blog articles can share the same tag.  When retrieving data would like to allow retrieval of all articles matching a tag.  In many ways very similar to the approach taken here at stackoverflow.
My normal mindset would be to create a many-to-may relationship between tags and blog articles.  However, I'm thinking in the context of GAE that this would be expensive, although I have seen examples of it being done. 
Perhaps using a ListProperty containing each tag as part of the article entities, and a second data model to track tags as they're added and deleted?  This way no need for any relationships and the ListProperty still allows queries where any list element matching will return results.
Any suggestions on the most efficient way to approach this on GAE?