I think I finally have a solution for using Rails migrations with Subversion branches. I've created a plugin, Subverted Migrations, that does the grunt work in merging migrations (similar to what svnmerge.py does for merging files). The plugin has two main functions:
1. Keeps your version numbers straight between branches, so you don't have conflicting version numbers when merging.
2. Keeps track of which migrations have been applied to the database, and applies merged-in migrations after merging even if newer migrations have already been applied.
Here's my blog post on the plugin, with a Chunky Bacon example to hopefully make things more clear:
The svn repo is:
I welcome any feedback, patches, bug reports, general whining and complaining, etc., as usual.