This little script will import all the assets contained in the folder you currently have selected, and all it's sub-folders. Just save this script in a folder in your project called Editor. Next, go to Assets > Reimport Folder and it will reimport the folder you currently have selected.

import System.IO;

@MenuItem( "Assets/Reimport Folder" )

static function ImportFolder()
{
    var realPath : String = Application.dataPath;
    realPath = realPath.Remove( realPath.Length - 6 ); 
    var selectedPath : String = realPath + AssetDatabase.GetAssetPath( Selection.activeObject );   
    var fileEntries : String[] = Directory.GetFiles( selectedPath, "*", SearchOption.AllDirectories );
    for( var file : String in fileEntries )
    {
       file = file.Replace( "\\", "/" );
       file = file.Remove( 0, realPath.Length );
       AssetDatabase.ImportAsset( file );
    }
}

I still don't get it. You copy the script in a txt flile, name it randomly and place it inside Editor folder in Unity installation dirrectory? It doesn't work for me. I hope that this script would solve my problem: when I import a big pack of assets linked together(by copying all the folders of the pack inside Assets folder), when Unity imports all, it crashes. If I import only parts of assets, all the links are broken.

i needed this to reload screenshots that i use as savegame thumbnails. just saying that this did the job for me:

AssetDatabase.Refresh();

Why would you need to reimport?

Any directories within the resource folder can be accessed with Resources.Load() and obviously if you overwrite a resource, which you have pointed to in your could it will be automatically "reimported" anyway. If this isn't what you looking for, comment back :)