Like Seidio's 3500mAh extended battery for the EVO, the Mugen battery comes with a replacement back cover. Unlike the Seidio offering, however, this one at least tries to disguise the added bulk by keeping the back as smooth as possible.
All this streamlining comes at a price, though, as Mugen's battery is $96.95. Seidio's battery, you may remember, retails for $69.95 (cheaper at Amazon) and comes with an extra 300mAh packed into the battery.
Whether these stated capacities are even true is up to you and your battery testing equipment to determine, but the extended battery options are there if you want them.
Thanks, StreakFan!