In it's current state (0.1.0) the plugin is incompatible with Redmine devel. I'll attach the contents of a patch-file which solves the issue, though the chosen solution is not working while running in development mode. This is a TODO.
I've also fixed a minor error in the settings-partial, resulting in a correct display of the currently configured project custom-field in the settings-partial which I've also renamed to prevent collisions with other plugins.

Index: app/models/iframe_text.rb
===================================================================
--- app/models/iframe_text.rb   (revision 218)
+++ app/models/iframe_text.rb   (working copy)
@@ -1,9 +1,7 @@
-require 'net/imap'
-
 class IframeText
   
   def self.get_iframe_text(project)
-    # Find all of the projects that have enabled the "ticket emailer" plugin
+    # Find all of the projects that have enabled the "google calendar" plugin
     iframe_text = project.custom_values.detect {|v| v.custom_field_id == Setting.plugin_redmine_google_calendar['iframe_text'].to_i}        
     iframe_text = iframe_text.value if iframe_text    
     
Index: app/views/settings/_googlecalendar_settings.rhtml
===================================================================
--- app/views/settings/_googlecalendar_settings.rhtml   (revision 0)
+++ app/views/settings/_googlecalendar_settings.rhtml   (revision 0)
@@ -0,0 +1,10 @@
+<% string_fields = ProjectCustomField.find :all %>
+
+This will allow you to create a tab that references a custom iframe.  You will need to go to 'Admin -> Custom fields' and create the follow field of type 'Long text'.
+
+

+  iFrame Text
+  <%= select_tag 'settings[iframe_text]',
+                 '<option value="">(Select one)</option>' +
+                 options_from_collection_for_select(string_fields, :id, :name, @settings['iframe_text'].to_i) %>

+
Index: app/views/settings/_settings.rhtml
===================================================================
--- app/views/settings/_settings.rhtml  (revision 218)
+++ app/views/settings/_settings.rhtml  (working copy)
@@ -1,9 +0,0 @@
-<% string_fields = ProjectCustomField.find :all %>
-
-This will allow you to create a tab that references a custom iframe.  You will need to go to 'Admin -> Custom fields' and create the follow field of type 'Long text'.
-
-
iFrame Text
-<%= select_tag 'settings[iframe_text]',
-               '<option value="">(Select one)</option>' +
-               options_from_collection_for_select(string_fields, :id, :name, @settings['iframe_tab'].to_i) %>

-
Index: init.rb
===================================================================
--- init.rb (revision 218)
+++ init.rb (working copy)
@@ -19,16 +19,17 @@
 
 RAILS_DEFAULT_LOGGER.info 'Google Calendar'
 
-# Redmine ticket emailer plugin
+# Redmine Google Calendar plugin
 Redmine::Plugin.register :redmine_google_calendar do
   name 'Google Calendar Plugin'
   author 'Jim Mulholland'
   description 'A plugin to allow users to add a new tab with an embedded Google Calendar Iframe.'
-  version '0.1.0'
+  version '0.2.0'
   
-  # The data necessary to log into your email server.
-  # For this first rev, I am assuming you have an IMAP server
-  settings :default => {'iframe_text' => ''}, :partial => 'settings/settings'
+  # This plugin contains settings
+  settings :default => {
+    'iframe_text' => ''
+  }, :partial => 'settings/googlecalendar_settings'
 
   # This plugin adds a project module
   # It can be enabled/disabled at project level (Project settings -> Modules)
@@ -42,10 +43,11 @@
   menu :project_menu, :google_calendar, :controller => 'google_calendar', :action => 'show'
 end
 
-Rails::Plugin.class_eval do
-   def reloadable!
-     load_paths.each { |p| Dependencies.load_once_paths.delete(p) }
-   end
-end
-
-reloadable!
+## Commented-out for Redmine 0.9-devel compatibility
+## TODO Fix development-mode compatibility
+# Rails::Plugin.class_eval do
+#    def reloadable!
+#      load_paths.each { |p| Dependencies.load_once_paths.delete(p) }
+#    end
+# end
+# reloadable!