<p>Do I really have to learn Objective-C to develop solid Mac Apps?</p>

<p>As Mac users tend to use only applications that have a nice (native) GUI, i don't think that Mono and GTK+ or any Java GUI (Swing) will fit their needs.</p>

<p>There are projects like Cocoa#, PyObjC and RubyCocoa, but are they ready for primetime?</p>

<p>So do I really have to learn Objective-C ? 
I would prefer a dynamic language.</p>

Objective-C
is
a dynamic language, as far as the Objective-C parts go.  Here's a little summary article:
http://www.macdevcenter.com/pub/a/mac/2003/04/28/objective-c.html
The syntax is scary at first, but it grows on you.  I suggest biting the bullet and slogging through it.
If you want to work at a "real job" doing Mac programming with other people, you're going to need to know Objective-C (in my opinion, anyway).