I kept getting a segmentation fault from abstract_adapter.rb when trying to connect to my controller – some people have reported success rolling back to MySQL 5.0, but if you want to use 5.1, try using the libmysql.dll from InstantRails:

http://instantrails.rubyforge.org/svn/trunk/InstantRails-win/InstantRails/mysql/bin/libmySQL.dll

(via peterskim.org)