Apparently, there's a shortage of Molybdenum-99 isotope, as well as other isotopes, used in medical testing. Who knew? Apparently, the folks at the Society of Nuclear Medicine (SNM):
http://www.snm.org/index.cfm?PageID=7739&RPID=10
Another story on this issue:
http://www.dailyiowan.com/2009/06/19/Metro/11756.html