As many of you already know, the current firmware for the Nokia N900 (known as PR 1.1) has some controversies regarding the Qt
toolkit.

EDIT:
Now that PR 1.2 is out, this is quite obsolete, since all of the stuff works on it without hacks.

However, I'll leave the guide here for those people who are stuck with 1.1:

To those of us who don't know, I'll summarize the situation.

- The libqt4-* packages are Qt 4.5.3
- The libqt4-maemo5-* packages are Qt 4.6.2
- The apps in extras-devel require libqt4-* to be 4.6.2, which is not the case at the moment
- The newly released shiny Nokia Qt SDK also requires libqt4-* to be 4.6.2, which it is not
- Developers of apps who want to use 4.6.2 which work on current devices need to link to libqt4-maemo5-* but there is poor (if any) support to it

Okay, so the above stuff prevents any wannable Qt developer from developing seriously for the N900.
But today (at 3:00 AM) after some trouble, I found out a possible work-arund until the precius PR 1.2 is released.

So, basically, all we need to get stuff working is libqt4-* version 4.6.2, and what a surprise, it is possible to install it via the SDK repository.

If anyone finds this is unaccurate, I'll happily correct it.
Please, read the guide before starting to proceed with the steps.

Big thanks to Andy1210 who helped me discovering it and also bricked his device during the process!

IMPORTANT:
DO NOT upgrade any other packages from the SDK repository, as it will very much brick your device. (This is from experience.)

MORE IMPORTANT:
I don't recommend non-developers to do this.
And I don't take ANY responsibity for anything.

1. App manager, menu, "Application catalogues"
2. Hit "New"
3. Enter the following:
Catalogue name: maemo.org SDK
Web address: http://repository.maemo.org/
Distribution: fremantle/sdk
Compnents: free non-free
4. Exit app manager and start a root terminal.
5. Basically, that's it. If something goes wrong, try to make apt to fix it.
You may have to remove the "mp-fremantle-generic-pr" package (or your version of it) before the above to make it work, which is responsible for notifying you of new updates. You won't need it anyway. (The buzz around PR 1.2 in TMO will be enough, you'll notice when it arrives anyway.)
The immediate result:
All the updates that require the new Qt version will succeed. (Ukeyboard won't since it requires more packages from PR 1.2.)

And finally, developers rejoice!
The brand new Nokia Qt SDK will work with the N900.

This is a quite sufficent solution for development, and if you'd like to make your app available to the rest of the word, you can still compile the app from the command line by linking to the libqt4-maemo5-* packages.
(Although the Qt Creator IDE won't do this for you.)

I hope this will help everyone who is as annoyed by this as I was.