<blockquote>Gnome and kde have proven to be unreliable, and changing way too often</blockquote>
Since the Gnome 1.0 release in March '99, there has been *one* incompatible change, when 2.0 was released, in June 2002. Since then, no incompatible changes in over 8 years. Gnome 3 is (currently) scheduled for March 2011, and there's no reason to think that it will last for any less time than Gnome 2 did.
Similarly, KDE 1.0 was in July '98, 2.0 in October 2000, 3.0 in April 2002, and 4.0 in Jan 2008. While 1 and 2 were fairly short-lived, 3 was a lot more mature lasting for 6 years, and with a lot of the technologies in 4 still being built upon with newer minor releases at nearly 3 years in, I predict that 4 will be a longer-lasting base than 3 was.
And, of course, apps written for Gnome 1, and KDE 1, 2 & 3 should all still run fine on any current and future Gnome/KDE desktops. You don't have to rewrite your KDE3 app to KDE4 technology if you don't want to. You can keep developing it against the old libs for as long as you want.