Index: app/models/user.rb
===================================================================
--- app/models/user.rb	(revision 46)
+++ app/models/user.rb	(revision 50)
@@ -96,7 +96,13 @@
   def self.try_to_login(login, password)
     # Make sure no one can sign in with an empty password
     return nil if password.to_s.empty?
-    user = find(:first, :conditions => ["login=?", login])
+    # This does a case-insensitive lookup:
+    # - Uses the index (avoids table walks) if the user exists (maybe causes table walks on MySQL for non-existant users?)
+    # - Does not depend on the database adapter
+    #   (MySQL will always match on the first, as it does not support function indexes;
+    #   PostgreSQL will match on first or second, both times with index (standard or the one created by the 20100203232731 migration);
+    #   SQLite should always be small enough for table walks)
+    user = find(:first, :conditions => ["login=?", login]) || find(:first, :conditions => ["LOWER(login)=LOWER(?)", login])
     if user
       # user is already in local database
       return nil if !user.active?
Index: db/migrate/20100203232731_add_case_insensitive_user_index.rb
===================================================================
--- db/migrate/20100203232731_add_case_insensitive_user_index.rb	(revision 0)
+++ db/migrate/20100203232731_add_case_insensitive_user_index.rb	(revision 50)
@@ -0,0 +1,15 @@
+class AddCaseInsensitiveUserIndex < ActiveRecord::Migration
+  def self.up
+    # Migrates PostgreSQL databases only
+    # MySQL compares case-insensitive by default
+    if ActiveRecord::Base.connection.adapter_name =~ /postgresql/i
+      execute "CREATE INDEX index_users_on_lower_login ON users (LOWER(login));"
+    end
+  end
+
+  def self.down
+    if ActiveRecord::Base.connection.adapter_name =~ /postgresql/i
+      execute "DROP INDEX index_users_on_lower_login;"
+    end
+  end
+end