It seems to be your first pacakge in Fedora Extras, so the first review must be
done by you sponsor. I may only help you cleaning up the spec file :]

- use %{dist} in the Release tag instead of hardcoding 'fc4'.
http://fedoraproject.org/wiki/Extras/BuildRequests
http://fedoraproject.org/wiki/DistTag

I'm not shure whether it's still true but few people on Fedora Extras were
suggesting adding %{dist} after importing package info CVS.
- remove spaces after "Applications/Internet"
- MacOSX â Mac OS X (?)
- Alsa â ALSA
- you have removed dots from the %description fileds. Please revert your
changes. They're not allowed only at the end of a "Summary".
- I'm not a native English speaker but "Modules aRts for Kadu" sounds weird to
me. Maybe "aRts module for Kadu" would be better?
- according to this page â
http://fedoraproject.org/wiki/PackageNamingGuidelines#NormalPackages you should
avoid using underscore in the package name.
- this files could be marked as %doc:

%{_datadir}/%{name}/AUTHORS
%{_datadir}/%{name}/README
%{_datadir}/%{name}/ChangeLog
%{_datadir}/%{name}/COPYING
%{_datadir}/%{name}/HISTORY
%{_datadir}/%{name}/THANKS

- you can also pass to this options the desktop-file-install program:

 --add-category InstantMessaging

http://standards.freedesktop.org/menu-spec/latest/apa.html  

New:
SPEC: http://kadu.net/~nastian/kadu.spec
SRPM: http://kadu.net/~nastian/kadu-0.4.1-2.src.rpm

I can't avoid underscore in package name becouse it is module name.
Files:
%{_datadir}/%{name}/AUTHORS
%{_datadir}/%{name}/README
%{_datadir}/%{name}/ChangeLog
%{_datadir}/%{name}/COPYING
%{_datadir}/%{name}/HISTORY
%{_datadir}/%{name}/THANKS
are used by Kadu in About window so they must be in that path. If I put them
also to doc they will be duplicated in package.

Sorry that it took me so long but I was a bit busy recently :/
(In reply to comment #1)

> Maybe "aRts module for Kadu" would be better?

This also applies to the "Summary" fields ;-)

(In reply to comment #2)

> I can't avoid underscore in package name becouse it is module name.

OK.

> Files are used by Kadu in About window so they must be in that path.

Oh, I didn't know about it.

I missed few things:
- remove hardcoded path from the kadu.desktop path:

Exec=kadu -caption "%c" %i %m

should be sufficient.
- in Categories field you can add "Qt" and maybe "KDE":
http://standards.freedesktop.org/menu-spec/latest/apa.html
This site does not mention "X-KDE-Base" so I'm not shure whether it's valid
entry :/ (help)
- drop these lines:

Requires:       qt >= 3.2
Requires:       libsndfile >= 1.0.11

RPM automatically detects proper requirements. What is more, Fedora Core and
Extras (FC3+) provide appopriate versions of these libraries.
- don't include /usr/lib/debug in the devel package
- architecture dependent files (modules) are in /usr/share directory - that's
against FHS 2.3 standard
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRSHAREARCHITECTUREINDEPENDENTDATA
I would suggest contacting with upstream and urge them to move these files to 
/usr/lib/kadu
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLIBLIBRARIESFORPROGRAMMINGANDPA
- modules are not stripped

And one more thing which looks weird to me - rpmlint warns about too long line
in description. Maybe it counts non-US-ASCII characters as two characters?

why this spec does not include external modules?

ps: .so files was moved to /usr/lib month ago, but as it would break
compatibility in 0.4 line, it will be included in next major release (0.5)

Kadu 0.4.3 has been released some time ago.

Mariusz do you still work on this package?

If I would like to take over this package, I should create new bug or continue 
it here?

You should create a new request and close this one

(In reply to comment #7)
> You should create a new request and close this one
Yes, but... I have no permissions to change status of this bug.

Bug 199192 is new request for this package.

*** This bug has been marked as a duplicate of 199192 ***