We heard a rumor about this a few months ago, and now it seems it has been confirmed. AMD will be dropping the Athlon name for their K10 series and beyond, at least for the more powerful desktop class CPUs. The new chips will now be under the name "Phenom", and each variant will have its own tag like we see today. We'll have Phenom X2, Phenom X4 (for quad core CPUs), and Phenom FX. This includes the code-named Agena CPUs and likely future cores. The reasoning behind the new name? To express how sheer awesome they will be, apparently:

Phenom is pronounced as an abbreviated version of the word "phenomenal." AMD's guidance states that the company will promote its Phenom X4 and X2 processors under the premise that Phenom allows its users to "experience the Phenomenal."
Whether or not they truly are phenomenal is up in the air until we actually get our hands on the new CPUs, which is still a ways off. It seems that for lower end CPUs, entry level perhaps, Athlon may stick around, though they haven't said otherwise.