If you haven’t chosen to select your font style within your account because you were happy with the default, you might want to rethink that. AdSense is changing the default font used in the different ad units, based on which ad font performs best in each ad unit. The new defaults will be:
Arial: 728×90, 336×280, 120×600, 120×240
Verdana: 300×250, 160×600, 468×60, 250×250, 234×60, 125×125, 180×150
Times New Roman: 200×200
This will go live within the next couple of days, so you will have a bit of time before the new fonts come into play, incase you want to double check what your default fonts are.