Filip,
Just did a quick look at phpinfo() on my production and development servers and I get the same "no value" bit for tmp_upload_dir. However, I know that file uploads do work on both servers (dev. is Win98 Apache running php 4.0.6-dev and prod. is Debian Apache running php 4.0.5) so I doubt that is what's causing the problem in your case. If no path is specified for tmp_upload_dir, PHP defaults to the system's temp directory. The reason you don't see anything in /tmp is that the server deletes the tmp files when the script has finished running.
In your phpinfo() output, is the value for file_uploads set to "1"? If not, there's your problem right off. You need to enable file uploads in php.ini and restart Apache. You should also check to make sure that you have specified "multipart/form-data" as the enctype for your submission form:
<form method="post" target="somefile.php" enctype="multipart/form-data">
as this is required for file uploading. If file uploads are enable, the missing enctype parameter is the most like cause of your problems. Regarding the location of your php.ini file, the path should be displayed in the top section of the phpinfo() output (same section that show's your make options, etc). HTH,
Cheers,
Geoff A. Virgo