I keep getting HTTP errors when using the flash upload in WP 2.5 and 2.5.1. After some digging around a bit, adding the following to my .htaccess file solved it:

<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>

Hopefully this helps someone else out too…