This is a special case for our team, too. We just broadcast that there's a new migration, and try to get it checked in before all the other changes. In the event of a collision, the person with the last check-in just renames the migration. SVN renaming is easy (I am still recovering from post-traumatic stress related to many years of CVS use).
We had thought this might go away when we all started to work off the same central development database (assuming that the migrate task got its number from the schema version table), but it seems to be just scanning the migration files.
Tom