Only 13 months into his stint, McNabb has presumably spent most of his time trying to figure out how to help his company get rid of two of his three brands, something that likely hasn't built much confidence for McNabb in terms of job security. In addition to uncertainty about what the future holds for the premium channel (Cadillac and GM itself), we suspect that the job's responsibilities are ultimately probably not what he he had in mind.
McNabb came to GM after a second stint at Nissan – he had actually been with both Nissan and Infiniti for 20 years, before he jumped off to join Mercedes-Benz in 2005 as head of marketing. After moving over to become head of Benz sales, he returned to Nissan the summer of 2007.
There is no official word yet on where McNabb is headed or who will be his permanant replacement, but in the meantime, Stephen J. Hill, currently the general sales manager of the premium channel, will take over in the interim. Said a Cadillac spokesman, there has been "a lot of positive change here" under McNabb, "and we have a lot of product in the pipeline and so we're well positioned for the future."
[Source: Automotive News, sub req'd]