GAE - How to live with no joins?
"Load user, loop through the list of friends and load their latest blog posts."
That's all a join is -- nested loops.  Some kinds of joins are loops with lookups.  Most lookups are just loops; some are hashes.
"Finally merge all the blog posts to find the latest 10 blog entries"
That's a ORDER BY with a LIMIT.  That's what the database is doing for you.
I'm not sure what's not scalable about this; it's what a database does anyway.
Example Problem:
Entities:

User contains name and a list of friends (User references)
Blog Post contains title, content, date and Writer (User)

Requirement:
I want a page that displays the title and a link to the blog of the last 10 posts by a user's friend.  I would also like the ability to keep paging back through older entries.
SQL Solution:
So in sql land it would be something like:

select * from blog_post where user_id in (select friend_id from user_friend where user_id = :userId) order by date

GAE solutions i can think of are:

Load user, loop through the list of friends and load their latest blog posts.  Finally merge all the blog posts to find the latest 10 blog entries
In a blog post have a list of all users that have the writer as a friend.  This would mean a simple read but would result in quota overload when adding a friend who has lots of blog posts.

I don't believe either of these solutions will scale.
Im sure others have hit this problem but I've searched, watched google io videos, read other's code ...  What am i missing?