Will,
This is not a migration issue but rather what appears to be something specific with MySQL - Rails isn’t trying to create any table - it’s on the MySQL side. Google “errno: 150” and have a look at the possible problems. There seems to be a pile of things that can pop this error. The second hit on google appears to use show innodb status to get more information out of the system (you would need to attempt the ALTER TABLE manually and you would obviously need to be using innodb).
Sorry can’t be more specific but this appears to be a catch all error that don’t reveal much about why it occurs…other then Rails isn’t the culprit 
John W Higgins wishdev@gmail.com
