If you kill the running postgresql connections for your application, you can then run db:drop just fine. So how to kill those connections? I use the following rake task:
# lib/tasks/kill_postgres_connections.rake
task :kill_postgres_connections => :environment do
db_name = "#{File.basename(Rails.root)}_#{Rails.env}"
sh = <<EOF
ps xa \
| grep postgres: \
| grep #{db_name} \
| grep -v grep \
| awk '{print $1}' \
| xargs kill
EOF
puts `#{sh}`
end
task "db:drop" => :kill_postgres_connections
Killing the connections out from under rails will sometimes cause it to barf the next time you try to load a page, but reloading it again re-establishes the connection.