Need help with possible GPL violation

[Tim Kosse]

Hi,

I'm the author of FileZilla (http://sourceforge.net/projects/filezilla),
an GPL'ed FTP client for Windows.

I've a problem with a company which tries to sell FileZilla under a
different name, though they have released it under the GPL as well.
They have just renamed my program but have added some information that
'their' product has been derived from FileZilla.
The source code of their version is also available, so far it is all GPL
compliant.

But the problem is, that they have made it trialware with nagscreens on
startup and shutdown, you have to register it after 15 days of use or it
won't start any longer.

Is this allowed or does it get in conflic with section 4 of the GPL:
"[...] You may not impose any further restrictions on the recipients'
exercise of the rights granted herein. [...]

I'm already in contact with that company and they claim that they do not
violate this section of the GPL, because they are doing a really strange
trick: 
They are using a launcher which will display the registration screen on
startup and which will launch the actual app. And after the trial
expires, everyone can still run the original app without any
restriction. However, there is no information anywhere that the original
app can be started directly and the launcher tells you that you have to
register. So the majority (if not all) of the users will think that they
really have to register.

Also, this company comes with the argument that other companies like
Lindows and Red Hat are selling GPL software. But those companies don't
use such tricks like launchers or trialware. Also, they are distributors
and they have created at least some code on their own. And in case of
Red Hat Linux, it is freely available on their FTP servers. You pretty
much pay for the CDs, the manuals and the support rather than for the
software.

Please help me with this issue, does this company violate the GPL or is
this allowed? Is it valid to sell trialware GPL software?

Regards,
Tim Kosse