I have a tentative fix for the issue of using Rails migrations with  
Subversion branches. I wrote a plugin, announced here:

http://www.bradediger.com/blog/2006/11/subverted_migrations.html

It is available at:

svn://svn.madriska.com/plugins/subverted_migrations

The plugin changes Rails's schema versioning to use version strings  
instead of integers, so you can have a version of "1-12,16-19" for  
example (with the missing versions being in another branch to be  
possibly merged in later). It also patches the db:migrate task to  
handle this versioning, as well as modifying the generate script to  
keep version numbers unique across all branches and trunk.

It is in alpha right now, but it is working well enough for me.

Is this something that would be a useful addition to Rails core? I'd  
appreciate some feedback / criticism.

Thanks,
Brad