Hi @janoChen:
Are you sure that code it running and not some other code? I just traced through the code and the only way it’s going to do that is if you have a plugin or code in your theme that is hooking one of the following hooks: 'the_excerpt', 'get_the_excerpt', 'wp_trim_excerpt' or possibly removed the 'get_the_excerpt' hook that calls wp_trim_excerpt().
What plugins or other code to you have installed?