Anyways, here's the preresults of my research:
First comes the free modellers:
argoUML - Worth a try, though it doesn't support UML 2.0
umbrello - Check that you have the latest (2.0) and greatest version installed. It should have some improvements to the previous 1.5x series that I recall having some usability problems and instability some years ago.
Violet - Worth a try. I suspect it to be a bit enchaned version of DIA for UML-modelling.
Gaphor - Seem like I nice project. Supports UML 2.0 and looks nice. It might lack in functionality as it's rather recent project (perhaps a year old product..)
UMLet - Looks nice. Can't really say anything about it's functionality, 'cos the homepage had no features section.
NetBeans UML Project - Migth eventually be somthing, though it seems, it is still in very early development stage. I suppose trying it wouldn't hurt, though...
Now follows the commercial editors
Poseidon - Heavy but should be great (at least on paper). I remeber having lots of problems with it (the free version) a few years ago though... The price isn't that huge, so actaully purchasing it might not be out of the question. I just have to ask my self do I really need it that much.
Omondo - Should be great application. At least the price of 4 digits suggest that.. also has a free version so that could be checked (eclipse plugin).
Visual Paradigm - Another entreprice level UML-modeller. Also has a free edition. Prices vary from free community edition to 4-digit enteprice edition and anything in between. (plugins for several different ides)
As a side note, here's a couple of usefull link about UML-modelling:
It's about agile model driven development (AMDD)
and IBMs site covering the UML basics.
and as a bonus: A "longish" (32 pages) pdf with title: What's new in UML 2.0
"Some" censorship to the site has been commited..