If Apple were to develop an add-on that provides a TV tuner and assorted inputs, TV capability would be something you could easily have with any of Apple's computers.
So doing this in concert with a 40-inch iMac would pretty much give you the oft-rumoured Apple TV. If you used the computer bits from the entry iMac in the 40-incher, you could easily bring the product in at less than $2,000 US.
Another way to go would be to still have an all-in-one form factor with the inputs and tuner incorporated into the 40-inch iMac and offer the add-on to get that functionality added to any Apple set-up.
It wouldn't take much for Apple to offer far more functionality in a television than the competition because the company has been building a monitor/computer all-in-one for years but unlike other computer makers Apple has really done a great job of building up content delivery via iTunes.
It would not, though, be a bad idea as well for Apple to offer a standalone 40-inch monitor. Take that device and combine it with a Mac Mini. I could envision a small Thunderbolt add-on that provided assorted inputs, multiple HDMIs, etc. Most of us now use satellite or cable boxes for tuning so the built-in tuner on TVs is a useless piece of tech that is a remnant from the past. I'm surprised that we've not seeing more homes with large monitors instead of full-blown TVs though that's probably because such devices are not what you are offered by your major retailers.
So Apple might possibly take on the TV segment not just with the introduction of a set but rather an assortment of devices that are geared towards that segment. A stand-alone 40-inch monitor (including various inputs) and a 40-inch all-in-one based on the iMac, possibly a Thunderbolt add-on that gives any Mac set-up full-blown TV capability.
In terms of pricing, what would be possible would be $1,499 for the 40-inch monitor, $1,999 for the 40-inch all-in-one, $149 for the Thunderbolt add-on. If, for example, one already had a Thunderbolt Mini, adding the monitor could be all that would be needed to have a home theatre good to go. If your cable box handled tuning and the monitor featured enough inputs, you'd be good to go for $1,499. For those looking to get a system up from scratch, the 40-inch all-in-one would provide a clean, uncluttered option. If one wanted to add TV capability to an existing Apple set-up, i.e. not pick up a new monitor, the add-on would be the answer.
The advantage of this approach is that Apple would have a lot of customers buying into its TV vision, not just the ones in the market for a TV alone.