After a long silence, we've finally released Checkvist Pro plan. The detailed post about this is available.
With this release, I've also upgraded Rails to the version 2.3.3 and migrated Mysql database from MyISAM to InnoDB engine (because we've had problems with data consistency when transactions were interrupted).
So far so good, performance haven't become worse. And I'm pretty satisfied with upgrade of the app engine from mongrel cluster to Nginx Passenger.
Now will focus on the daily work and on adding more features to Checkvist.