Qt has great cross-platform promise. It's not quite there yet, writing cross-platform mobile apps is "write once, #ifdef everywhere" at the moment. But if you're targeting mobile app development for Nokia devices, learning Qt is certainly the place to start. Qt with e.g. Qt Mobility additions is the greatest common denominator between the platforms.
If you're planning to do simpler widget applications, go to HTML5 instead. It's even more cross-platform. (Noticed the nokia-wrt tag: Forget about WRT. It's been discontinued.)
As for the platform lifecycles, it's just normal evolution. New platforms will replace older ones in high-end devices. Older platforms don't die but just end up being used in cheaper lower end devices. Thus Symbian won't die any time soon. In the foreseeable future, there will still be huge numbers of Symbian-based devices in the market, capable of running Qt apps compiled for Symbian. Heck. even the older Series 40 and Series 30 platforms are still around and doing well in their segments, though Qt doesn't run on them.
In the case of Maemo and MeeGo, MeeGo will replace Maemo (technically MeeGo is sort of Maemo 6).
In the case of Symbian evolution, Nokia has ditched the Symbian^n model as of Symbian^4 and moved to a continuous, rolling updates model. So there will just be the Symbian platform, not many of them.