Problem:
Wordpress version 2.5 wp-mail.php cuts off subjects and content containing german umlauts.
Solution:
We can fix this by adding utf8_decode to wp-mail.php
Add this code:
Line 65
$subject = utf8_encode($subject);
Line 154
$content = utf8_encode($content);
Status:
a TRAC report has been filed under http://trac.wordpress.org/ticket/6788