the_excerpt doesn’t work in 2.5.1?

I am working on a theme on a 2.5.1 when I noticed that
the_excerpt is not working like normally it should. I thought it was my coding, so I tested the same theme on 2.5 – the_excerpt works OK there.

I also checked using the Default theme in 2.5.1 – I changed the_content inside the index.php into the_excerpt – still showing the whole content instead of the excerpt. I already re-installed 2.5.1 and the problem is still there.

Anyone having the same problem?