According to a report out of Taiwan, Intel is apparently looking at Sanyo's and BAK's 16650 cylindrical battery as well as prismatic lithium-ion batteries.
The cylindrical battery seems to be the device of choice due to its low cost, but its diameter of 16 mm makes it a challenge to be built into Ultrabooks, which, by Intel's definition, need to be less than 21 mm thick. A problem may also be that the market for these batteries is controlled by Sanyo and BAK and the supply is very limited. Prismatic are more expensive, but are only 5 to 6 mm thick, while using a greater footprint of 60 x 80 mm.
Digitimes noted that Acer is using the 16650 cylindrical battery in its Ultrabook while Lenovo is using a prismatic battery in its IdeaPad 530.