Since the N900 has no physical button to call up the task manager, you need to exit the full screen mode of an application in order to tap the top left corner of the screen and bring it up. This is a bit of an annoyance, although you can bring it up with the Ctrl + Backspace shortcut incase you are browsing with the keyboard slidout.
But you will have to agree it would be far more convenient if we could simply press a physical button to bring it up. Heikki Holstila certainly felt so because he created a small utility called ‘Camkeyd’ that turns the camera key into a dedicated shortcut for the task manager.
You can still use the camera key to launch the Camera by completely depressing the camera key. The application is a very simply install from the task manager and has no UI. You can install it to enable the shortcut and uninstall it to disable it. You will find it under the System head of the Application Manager on your N900 if you have the Extras-Devel repository enabled.
Here is the warning from the wiki at maemo.org.
The software hosted in extras-devel is most likely not ready for end users! PLEASE PLEASE PLEASE don’t play with it unless you really know what you are doing.
Developers upload the newest version of their software to Extras-devel. From there the packages go through an automatic and human Quality Assurance process. This is a repository for developers and regular contributors of specific software projects. If you want to play with extras-devel software you need to be prepared to feel some pain sooner or later.
The application seems to be working wonderfully for me, however your mileage may vary. Please proceed with caution when dealing with applications from extras-devel.