myDNA notes that "scientists have discovered how blood-regenerating stem cells move from bone marrow into the blood stream. The finding has led to the development of a new chemical compound that can accelerate this process (called stem cell mobilization) in mice - which could eventually lead to more efficient stem cell harvesting for human use." One reason researchers are searching for other sources of multipotent stem cells is that bone marrow extraction is an unpleasant, expensive procedure. Making bone marrow stem cells come out for convenient collection from blood is an intriguing alternative to attempting to use skin or fat stem cells, for example.
Link: http://www.mydna.com/genes/genetics/news/resources/news/200507/news_20050727_stem.html