After more than a year of heavy and careless usage, dust speckles found their way to the sensor of my beloved Nikon D40. I discovered that unfortunate fact during my recent road trip, while visiting Arches NP. I attempted to solve the problem at the spot, and that made the whole situation even worse. I started off with one small dust speckle in the upper right corner of my photos, and I finished with two pronounced speckles located more or less in the center of all the photos. Removing them from my pictures was a major effort, and in some cases (e.g. when they happened to show up on somebody's face or on the horizon lane) it was impossible to remove them without ruining the photo.
Do not repeat my mistake and do not attempt to clean your camera if you do not know how to do it and if you do not have appropriate equipment with you! I definitely learned from my mistake.
After coming back home, I googled "DSLR/Nikon D40 sensor cleaning", read hundreds articles and watched several videos on the subject, bought all necessary equipment to do the job, and finally, today, I cleaned the sensor on my camera. It went smoother than I had expected, and I am very happy to report that my camera is fully functional again. So you can expect thousand of new photos soon :)