Index: media.php
===================================================================
--- media.php	(revision 18464)
+++ media.php	(working copy)
@@ -1568,6 +1568,21 @@
 	<span><input id="cancel-upload" disabled="disabled" onclick="cancelUpload()" type="button" value="<?php esc_attr_e('Cancel Upload'); ?>" class="button" /></span>
 	</div>
 	<p class="media-upload-size"><?php printf( __( 'Maximum upload file size: %d%s' ), $upload_size_unit, $sizes[$u] ); ?></p>
+	<?php
+		$wp_multiselect_key = false;
+		if ( strpos($_SERVER['HTTP_USER_AGENT'], 'Mac') !== false ) {
+			$wp_multiselect_key = 'CMD';
+		} elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Windows') !== false ) {
+			$wp_multiselect_key = 'CTRL';
+		} elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Linux') !== false ) {
+			$wp_multiselect_key = 'CTRL';
+		}
+		if ( $wp_multiselect_key ): ?>
+	<p class="multi-file-upload"><?php
+		printf( __(' <strong>Note:</strong> To select multiple files to upload you can press the <kbd>%s</kbd> key within the file selection popup.' ), $wp_multiselect_key );
+	endif; ?></p>
+	<?php unset( $wp_multiselect_key ); ?>
+
 <?php do_action('post-flash-upload-ui'); ?>
 	<p class="howto"><?php _e('After a file has been uploaded, you can add titles and descriptions.'); ?></p>
 </div>