Google is updating default font faces in AdSense, associated with specific ad formats, that’ll only affect ad units in Latin-based characters which’have 'AdSense default' selected as the font face. Currently, most ad units set to the AdSense default font face’re displayed in Arial, with a few in Verdana. Here’re the new default font faces for ad units:

Arial: 728x90, 336x280, 120x600, 120x240
Verdana: 300x250, 160x600, 468x60, 250x250, 234x60, 125x125, 180x150
Times New Roman: 200x200