> suppose, I write a commercial (shareware) program, that 
> uses the (unmodified) commandline version of httrack. 
This 
> should not violate the GPL, right?
"These requirements apply to the modified work as a whole. 
If 
identifiable sections of that work are not derived from the 
Program, 
and can be reasonably considered independent and separate 
works in 
themselves, then this License, and its terms, do not apply 
to those 
sections when you distribute them as separate works. But 
when you 
distribute the same sections as part of a whole which is a 
work based 
on the Program, the distribution of the whole must be on 
the terms of 
this License, whose permissions for other licensees extend 
to the 
entire whole, and thus to each and every part regardless of 
who wrote it. "

As a GUI wouldn't be independent (as it would not work 
without the main code), I suppose it is not 100% compliant 
with the terms of the license.. but as I'm not a lawyer, I 
really can't tell more.