From Thinkgeek:

Artoo’s ready to protect up to 20 oz. of your caffeinated beverage of choice. You see, his head serves as a removable lid so your liquid stays just the right temperature longer. Droids are useful that way. Also, one of his attachments is probably for frothing milk. But we haven’t checked. We figured that’d be impolite.

[Star Wars R2-D2 20oz. Lidded Mug]