What helps nowadays more in Maemo is the Qt, it makes the development of native applications a magnitude easier and more efficient. Instead of tinkering a lot with the user interface, now you can build one even with Qt Creator / Qt Designer.
Qt for Maemo 5, that Antonio Aloisio and Kate Alhola among others made it possible with their long work of hildonizing Qt, is now available as beta now from the Qt-labs. This technology luckily became officially supported from the roots of community supported port and hopefully this will get now even greater audience than before and attract more Maemo developers and prospective Maemo developers than before.
While Gtk is available and Maemo 5 applications have been done with Gtk, I would recommend to use Qt in the new future projects. PeterMaemo is saying that Qt is "forward-compatible technology on Maemo for years to come...", I want to believe in that because the most wise thing to do is now stick with Qt for years to come to have best possible backwards compatibility for UI applications.
Recently there have been interesting development on native iPhone apps which utilize the power of the CPU (which would not be possible with some runtime without the CPU of the device being a supercomputer), like X-plane flight simulator, a controller for RC aircraft that streams video to the device and uses augmented reality to make a game on top of the cake, Voice Band where one can sing notes in a sequencer which translates them to notes and plays back with instruments so one can do convincing sounding compositions on the road without any musical instruments other than one's voice etc.
I would be eager to see this kind of apps to emerge on Maemo. Maemo 5 has OpenGL ES 2.0 support, Qt, and a native application development environment. I think that is very cool, and one can do really creative and innovative things with it. If somebody will do X-plane quality simulator for N900, I will certainly even willing to pay for it. Same goes for Voice Band, I would eager to use it when I am not front of my iMac which is stationary in the corner of our living room - you know, you can't really control when and how you compose, the ideas just pop in the head by itself in uncontrolled manner and they are gone (at least happens in my case) if they are not right away utilized. And I would like also augmented reality apps for the Maemo. Maemo has great camera and it is the most open phone out there, so I would think it would be the ideal place to hack this kind of things. And what's more, there is even TV-out on the Nokia N900, I was thinking that one could connect data glasses to the TV out potentially and the image from camera and some computation could be used as augmented reality - not just in someday in the future, but right now today on Maemo 5! If you have some great idea, now it is a good time for pursuing it. Just remember to use Qt as UI toolkit and it has a good chance to survive also the future incarnations of the platform (you may need to do modifications to places where you use the lower layer, but if you do it wisely and in portable manner, you might not be so unlucky with future generation of Maemo and the Maemo devices).
A long time ago I was doing a loudspeaker box design program with Qt (actually that was before I even joined Maemo). The project unfortunately almost died. However, Kate found one day a backup from one hard drive and she uploaded the code to our home gforge. Luckily it worked out of the box with Qt 4.5.x on Today's Ubuntu and possibly will also work with Qt 4.6. And luckily enough, the UI was actually designed with Qt-designer, so basically it is easily replaceable, and I can possibly produce a Maemo-version. Loudspeaker box design program may be niche, but since the current availability of them for other platforms than Windows is poor, it could be possibly cool to make this application a reality. It currently calculates correctly just closed box and the vented box calculation is broken. I will look if I can make it work with the vented boxes as well and then it could become somewhat useful compared to otherwise nothing available. It will not have all the features of e.g. WinISD initially, but it is cross platform and will run on Mac, Linux and Maemo, so that should be something. The cool thing about Qt is that you don't need any high level language that would potentially cripple the features and performance to achieve cross platform compatibility (the calculation algorithm takes some time even on desktop machine, so it would be possibly too heavy for some Java app or similar). I think I will give it a try because it turned out that I will need this little software myself right now, so do it yourself is the best help yourself. It will be free GPL software.
I hope many people get inspired with the vast possibilities of the great Maemo platform and start making really cool things that utilize the CPU and GPU of Maemo device fully. Anybody can for example write a simple calculator app with some high level language, or maybe like somebody wrote the silly "I am rich -app" for iPhone, even entirely on QML, but when there are dozens of little calculator apps and silly apps which have no usefulness after 1 minutes of trying, it starts to be no point to be yet another on the sea of many. My recommendation: find something that does not already exist, get yourself an inspiration, aim for superhigh quality, and just go for it! Maemo is cool, but it is even cooler with your app, and your audience will love your creation!