A hospitalist is not a critical care doctor unless he has obtained training and accreditation for it, however most IM training programs encorporate about 6 months of ICU training, and just through the breadth and depth of medical training are optimally suited to advanced training in the ICU. Depending on the hospital, intensivist physicians may not always be available and the ICU duties may swing to the hospitalist du jour. An open ICU model is described as one where the floor physician continues primary control and care of his patient in the ICU, often times obtaining the service of a staff intensivist for complex cases or vent management. A closed ICU, which has been shown to be superior in terms of cost savings and overall care, involves handing over the care of one's patient to the covering intensivist and his team/colleagues. This is probably the the model which you have been observing.
Right now, Pulm/CCM docs are in demand, esp. because they can staff closed ICUs. This is ultimately a cost cutting move as ICU care is expensive and the use of the intensivist often streamlines use of resources. However, don't be surprised if you do see hospitalists doing ICU care...there just aren't enough CCM trained docs out there right now.
