Just to share my experience.
it works for me for the latest released (phpr build 9769), i.e. export to EXCEL 2007 xlsx file.

but i used to have the similar problem during phpr 6 beta version.
The strange things was when I had the error,

i checked out FAQ the website for phpexcel which bundled with phpr:
http://phpexcel.codeplex.com/wikipage?title=FAQ&referringTitle=Requirements
It mentioned what the sergey said, need the ZIP library.

and i check my PHP Version 5.3.5 on windows, it does not have the line at all:

;extension=php_zip.dll
So i gave up on that, thinking recompling to include the zip lib was a hassle.
But strange things is, now it works for me, export to excel 2007 with xlsx file.

I check gain on my web server and dig into the php.ini, still there isn't the extension line.
In the FAQ of the PHPEXCEL website, it says:

The php_zip extension on Windows coming with PHP 5.2.6 and PHP 5.2.8 contains an error when creating ZIP files.

See http://bugs.php.net/48763 for details. The version that can be found on http://snaps.php.net/win32/php5.2-win32-latest.zip

should work at all times.
Alternatively, upgrading to at least PHP 5.2.9 should solve the problem.
If you can't locate a clean copy of ZipArchive, then you can use the PCLZip library as an alternative when writing Excel2007 files, as described above.

And may be is due to my version php 5.3.5 ?
When i echo my phpinfo, i have some of these which i do not know it is the required zip lib:

Registered PHP Streams: php, file, glob, data, http, ftp, zip, compress.zlib, compress.bzip2, https, ftps, phar

HTTP_ACCEPT_ENCODING: gzip, deflate

BZip2 Support Enabled
zip

Zip enabled

Extension Version $Id: php_zip.c 305848 2010-11-30 11:04:06Z pajoye $

Zip version 1.9.1

Libzip version 0.9.0
zlib

ZLib Support enabled

Stream Wrapper support compress.zlib://

Stream Filter support zlib.inflate, zlib.deflate

Compiled Version 1.2.3

Linked Version 1.2.3
Directive Local Value Master Value

zlib.output_compression Off Off

zlib.output_compression_level -1 -1

zlib.output_handler no value no value
Phar

Phar: PHP Archive support enabled

Phar EXT version 2.0.1

Phar API version 1.1.1

SVN revision $Revision: 305584 $

Phar-based phar archives enabled

Tar-based phar archives enabled

ZIP-based phar archives enabled

gzip compression enabled

bzip2 compression enabled

OpenSSL support enabled
Phar based on pear/PHP_Archive, original concept by Davey Shafik.

Phar fully realized by Gregory Beaver and Marcus Boerger.

Portions of tar implementation Copyright (c) 2003-2009 Tim Kientzle.

i am not sure above which one actually works for the required ZIP.

But i just want to say, i don't have that: ;extension=php_zip.dll

but it works for me.
Hope someone can confirm what to lookout for in the phpinfo().
acp

Just to share my experience.
it works for me for the latest released (phpr build 9769), i.e. export to EXCEL 2007 xlsx file.

but i used to have the similar problem during phpr 6 beta version.
The strange things was when I had the error,

i checked out FAQ the website for phpexcel which bundled with phpr:
http://phpexcel.codeplex.com/wikipage?title=FAQ&referringTitle=Requirements
It mentioned what the sergey said, need the ZIP library.

and i check my PHP Version 5.3.5 on windows, it does not have the line at all:

;extension=php_zip.dll
So i gave up on that, thinking recompling to include the zip lib was a hassle.
But strange things is, now it works for me, export to excel 2007 with xlsx file.

I check gain on my web server and dig into the php.ini, still there isn't the extension line.
In the FAQ of the PHPEXCEL website, it says:
And may be is due to my version php 5.3.5 ?
When i echo my phpinfo, i have some of these which i do not know it is the required zip lib:
i am not sure above which one actually works for the required ZIP.

But i just want to say, i don't have that: ;extension=php_zip.dll

but it works for me.
Hope someone can confirm what to lookout for in the phpinfo().
acp