Mark Jaquith has a snippet of code at http://markjaquith.wordpress.com/2009/12/14/excluding-your-plugin-or-theme-from-update-checks/ however there is an edge case it misses, causing notices and warnings when wordpress.org cannot be reached. Happens more on local developer and QA environments rather than production but should still be fixed

gistfile1.php

<?php

	/**
	 * Exclude from official repo update check.
	 *
	 * @link   http://markjaquith.wordpress.com/2009/12/14/excluding-your-plugin-or-theme-from-update-checks/
	 *
	 * @param  array  $r
	 * @param  string $url
	 * @return array
	 */
	public function http_request_args( $r, $url ) {

		if ( 0 !== strpos( $url, 'http://api.wordpress.org/plugins/update-check' ) )
			return $r; // Not a plugin update request. Bail immediately.

		if ( $r['response']['code'] != 200 ) {
			// this is a failed request! We cant modify the results if the results timed out/failed
			return $r;
		}

		$plugins = unserialize( $r[ 'body' ][ 'plugins' ] );
		unset( $plugins->plugins[plugin_basename( __FILE__ )] );
		unset( $plugins->active[ array_search( plugin_basename( __FILE__ ), $plugins->active ) ] );
		$r[ 'body' ][ 'plugins' ] = serialize( $plugins );

		return $r;
	}