The problem with the data stream from zimbra to aspell.php was not present in zimbra 4.0.2.
In fact, we have been using spanish spellcheck for 1 month until 4.0.3 upgrade:

http://bugzilla.zimbra.com/show_bug.cgi?id=11746

By the way, there is a difference between 4.0.2 and 4.0.3. In 4.0.2, default encoding was UTF-8, but now it has been change to ISO-8859-1... perhaps this is the source of the problem.