The office tower is being built on the eastern half of the block:
Here are the north/south elevations. The residential tower will be going up in the SW corner of the lot where the Claudia Johnson building is. Having that facade being the entrance to the residential tower will be pretty cool. But it is very telling that Trammell Crow is delaying the start of the residential tower.
ftp://ftp.ci.austin.tx.us/ATD_AULCC/...0Blk_PLANS.pdf