Silly me.
Reported in apache's error_log:
[Wed Nov 22 21:51:44 2006] [error] PHP Warning: File upload error - unable to create a temporary file in Unknown on line 0
Forgot to set upload_tmp_dir in php.ini :\
;;;;;;;;;;;;;;;;
; File Uploads ;
;;;;;;;;;;;;;;;;
; Whether to allow HTTP file uploads.
file_uploads = On
; Temporary directory for HTTP uploaded files (will use system default if not
; specified).
upload_tmp_dir = /tmp
; Maximum allowed size for uploaded files.
upload_max_filesize = 2M
You might try doing the same if you've got a similar setup.