OpenCV Object Detection Howto:
By Florian Adolf ( robotik [at] inflomatik [dot] de )
How-to build a cascade of boosted classifiers based on Haar-like features
Download the pdf file which explains 'how to' in windows..: OpenCV Object Detection
The tool "objectmarker" mentioned in the tutorial can be obtained here.
However, you should favor this improved version available here (cached doc and code). It is supposed to build under Linux and includes a good user documentation.
To convert the directory with subfolders 1..n to a xml file use the haartraining archive provided in the files section of the yahoo group.
Trained classifiers and papers are available from Prof. Lienhart here.