You have to create extra bootstraps for each your module.
For example:
[PHP]
//application/modules/admin/Bootstrap.php
class Admin_Boostrap extends Zend_Application_Module_Bootstrap
{
//can be empty
}[/PHP]

Then your Admin_Form_NewCategory will work.

For more info, look there: Modular applications in ZF 1.8 mzBlog

I too was struggling with this and I put the issue aside to deal with other problems. Then I started implementing Module-level ini configurations following this post :

Zend Framework: module config | vandenbos.org

and discovered the solution to getting Module-level Bootstrap files to actually load.

The answer to the problem is not in any of the documentation I have read regarding Module implemention in Zend Framework. But all you need to do is to add 'resources.modules[] = ' to your application.ini file. This line will then enable the loading of your module Bootstrap.php files.

Remember to name the module-level Bootstrap class after your module. eg.
'class Admin_Bootstrap extends Zend_Application_Module_Bootstrap' or you will get an error about 'Cannot redeclare class Bootstrap'.

Hope this helps.