You can get mugs in all shapes and sizes, including platonic solids. Well, somewhat deformed ones, in the case of this "20-sided" mug from ThinkGeek, but I think a cup's primary job should be to ably hold liquid, rather than serve as a crit-dealing die.
Made of ceramic and "exclusive" to ThinkGeek, the "Critical Hit D20 Mug" also comes with a lid to help retain heat (and keep giant spiders from climbing inside). It can hold 350mL, which is more than sufficient for a whopping big cup of coffee, or whatever beverage you care to pour inside it.
I'm not entirely sure where the "critical hit" part of the name comes from -- sure, there's a up-facing 20, but that's a cocked dice if I've ever seen one. Any dungeon master worth his salt would ask for a re-roll.
Critical Hit D20 Mug [ThinkGeek]