If you're in an area where Comcast "upgraded" your service so that all TVs require some sort of box, you're almost certainly out of luck.
The new basic digital boxes they're handing out require your TV to be tuned to one channel only (3 or 4 for example) and the box does all the work. That WILL NOT work with the Sling Tuner. If you try it, you might be able to sling on channel 3 only, and you have to use the box to change channels. No way around it either. I suppose in a pinch you could manually set the channel on the box then sling in to watch it, but it would have to be done ahead of time from home at the box and that's the total extent of the control you'll have.
I don't have the specific documentation on this, but as I recall the Tuner model only has an analog NTSC tuner on board. This works with analog cable and the antennas grabbing the now turned-off broadcasts out of the air. Digital anything won't work. CATV traditionally has included analog service for basic/expanded basic cable channels (2-99) even with customers who have digital packages. That's what Comcast is turning off in certain areas. Without that, the tuner is basically useless. You can't even hook up an antenna anymore now that broadcast has moved over to ATSC.
This problem affects all models of Slingbox to some degree, although ones that can eat ATSC and QAM signals handle it with a little more grace.
Perhaps this is the direction that cable TV needs to go. It might end up being a good thing, newer technology, standards and such. In the meantime, it frankly an annoyance.
Hope this helps,