The Sigma APO 50-150mm f/2.8 EX DC OS HSM lens was announced back in February, 2011. The lens was initially supposed to start shipping in October, 2011 but then it was further delayed till 2012.
In September the Sigma APO 50-150mm f/2.8 EX DC OS HSM was still listed on Sigma's website:
Currently the lens is completely removed from Sigma's website:
and it's not even included in the telephoto zoom section:
The older version Sigma 50-150mm f/2.8 EX DC APO HSM II is listed as discontinued:
