I went ahead and installed the Qt SDK from http://jenkins.taulabs.org/distfiles/
It installs two flavors/skins/themes/whatever of QtCreator, one for generic Qt development, one for development for Nokia devices. The latter comes with the toolchain for harmattan installed.

Then I used the SDK Maintenance Tool to install the Maemo toolchain! It's in the list of available add-on packages and installation went fine.

I then had to manually add the Maemo 5 target to QtCreator, but that was easy:

1. Start the QtCreator for Nokia devices
2. Go to Tools -> Options ->Build & Run
2.1 In the tab Qt Version, Add... a new one; navigate to ~/QtSDK/Maemo/4.6.2targets/fremantle-pr13/bin/qmake
2.2 In the tab, Add a GCC-based compiler; then below add compiler (~/QtSDK/Maemo/4.6.2/toolchains/arm-2007q3-51sb6-gdb71-arm-none-linux-gnueabi_linux/bin/arm-none-linux-gnueabi-c++) and debugger (~/QtSDK/Maemo/4.6.2/toolchains/arm-2007q3-51sb6-gdb71-arm-none-linux-gnueabi_linux/bin/arm-none-linux-gnueabi-gdb)

Optional, to execute directly on device:
3. Go to Linux Devices
3.1 Add... a Device with MADDE support (Fremantle should be in the list), start the wizard, select Maemo 5/Fremantle from the drop-down, set appropriate values for the other fields
4. On your N900, apt-get install mad-developer
5. Make sure you can access your N900 using SSH: http://talk.maemo.org/showthread.php?t=41547

6. Profit.