I purchased my AMD X2 based system in the beginning of the year and a couple of months ago (after upgrading Media Center), my system stopped seeing the second core. Task Manager only provided status on one processor and Device Manager only showed one Processor. It was strange because the it properly identified one core as an "AMD Athlon(tm) 64 X2 Dual Core Processor 4200+", but it only showed one.
I just found David Fleishman's blog entry about a similar issue where he posts a link to an updated processor driver. Once installed, it requested a system reboot and now both cores are back and working.