"Dr. iPod" is the nom du hack of a Hungarian iPhone repairman and tinkerer, who — frustrated at the battery drain of his iPhone — hit upon the novel approach of a second, external battery, itself housed in a second, piggybacked iPhone case he just happened to have lying around. (His blog gives step-by-step pictures of the process, and [unlike the rest of his site] is at least in English.)
While admitting the "HD" (for "heavy duty") mod adds millimeters and grams to the iPhone's size (not to mention hours to the charging time), he estimates that a day's moderate usage still leaves him with 96% battery capacity, as opposed to 60-70% with the single power source. (Voided warranty? What's that?)
No word on whether the Doc will offer this "iPhone-kenstein"/"Thing With Two Brains" mod outside of his native country, if at all.