A custom solution like the other comment suggests is probably best, but if you really want to use a module, your best bet is ThemeKey. As of 5/23/11, it has a stable release.

Considering that the change is for a limited period of time, and that implementing such code is not difficult (I am not talking of changing the core code, which is not a good idea to do), I would suggest to implement a custom module that changes the theme being used for specific pages.

It is enough to implement hook_custom_theme() as follows:

function mymodule_custom_theme() {
  if (arg(0) == 'event2011') {
    return 'the theme to use for that page';
  }
}

If you would need to change the theme only for http://example.com/event2011, but not forhttp://example.com/event2011/node/123, then the code should be changed to

function mymodule_custom_theme() {
  if (arg(0) == 'event2011' && !arg(1)) {
    return 'the theme to use for that page';
  }
}

As for using the theme callbacks in the definition of a menu callback, the documentationsays:

As a general rule, the use of theme callback functions should be limited to pages whose functionality is very closely tied to a particular theme, since they can only be overridden by modules which specifically target those pages in hook_menu_alter(). Modules implementing more generic theme switching functionality (for example, a module which allows the theme to be set dynamically based on the current user's role) should use hook_custom_theme()instead.

Alternatively you can use the new theme callback option of the menu systemhook_menu_alter() as outlined below. p.s Checkout hook_menu() for more details on thetheme callback

<?php
/**
* Implements hook_menu_alter().
*/
function mymodule_menu_alter(&$items) {
  // Set the theme callback function for all node pages. As per the
  // standard behavior for hook_menu() properties, this will be
  // inherited by all paths underneath node/%node as well, unless
  // they define their own theme callback.
  $items['node/%node']['theme callback'] = 'mymodule_default_node_theme';

  // Set a different theme callback for node edit pages, and pass
  // along the node object to this function so we can make decisions
  // based on it.
  $items['node/%node/edit']['theme callback'] = 'mymodule_edit_node_theme';
  $items['node/%node/edit']['theme arguments'] = array(1);
}
/**
* Defaults to using the 'some_theme' theme for node pages.
*/
function mymodule_default_node_theme() {
  return 'some_theme';
}

/**
* For editing page nodes, uses the 'some_other_theme' theme.
*/
function mymodule_edit_node_theme($node) {
  return $node->type == 'page' ? 'some_other_theme' : mymodule_default_node_theme();
}
?>

Additionally there is also an example of using the more traditional hook_custom_theme()

<?php 
/**
* Implements hook_custom_theme().
*/
function mymodule_custom_theme() {
  // check path using arg(0)
  // check $user
  // do whatever special checking you want and simply return theme key (name of theme folder most of the time)
    return 'special_theme';
  }
}
?>

Taken from: http://drupal.org/node/224333#custom_theme

You can simply use Context and use the theme to get applied when the path is /event2011/ you can simple set the path in condition of cotext and change the theme in the action of the content. This was you can very easily switch between themes on a site based on URL. Even works for mobiles ;)

How to switch theme depending on user role:

Create a custom module and copy & paste the following:

<?php
/**
 * Implementation of hook_init().
 */
function mymodule_init() {
  global $custom_theme, $user;
  if (in_array('my special role', $user->roles)) {
    $custom_theme = 'mytheme';
  }
}
?>

You have to replace:

mymodule => with your module name

my special role => with the name of the role that your users will need to have in order for them to see a different theme.

mytheme => with the name of the theme that you want to switch to