hook_menu implementation page not found error drupal 7.10
Try adding a 'title' to your return array.

I noticed that the D7 menu documentation lists 'Title' as required:

"title": Required. The untranslated title of the menu item.


Now there is a comment on the doc page that claims it is not required.

But That comment is from March 20, 2011. 

Perhaps the title is now finally required in a more recent release.

Edit:
Your code from comment:

/** * Implementation menu(). / */
function misite_menu() {
    $items = array(); 
    $items['a/main'] = array( 
             'title' =&gt; 'main',
             'page callback' =&gt; 'main', 
             'description' =&gt; t('Main front page'),
             'access callback' =&gt; TRUE, ); 
    return $items; 
} 

/* * Implementation hook_theme(). */ 
function misite_theme() { 
    return array( 'main' =&gt; array ( 
        'template' =&gt; 'main', 
        'variables' =&gt; array('title' =&gt; NULL), 
        ),
    ); 
}

function main() {
    $path = drupal_get_path('module', 'a'); 
    $title = t(''); 
    $build['mainelement'] = array( '#theme' =&gt; 'main', 
                                                '#title' =&gt; $title, ); 
    $output = drupal_render($build); 
    return $output; 
}


Several observations.
1) I think your inclusion of hook_theme could be causing needless grief. Try getting an example of hook_menu working independent of hook_theme. 

2) I wouldn't use 'main' as a page callback name; it may not be conflicting with another function elsewhere but I don't think it is a good idea regardless.

Try this:

function misite_menu() {
    $items = array(); 
    $items['a/my_main'] = array( 
             'title' =&gt; 'my_main',
             'page callback' =&gt; 'my_main', 
             'description' =&gt; t('Main front page'),
             'access callback' =&gt; TRUE
            ); 
    return $items; 
} 

function my_main() { 
    $output = 'HERE I AM'; 
    print($output); 
}


Then navigate to a/my_main and see if you can see "HERE I AM". If you can then you know your hook menu is working and that your problem is in your hook_theme.