Ok, I'll jump up on my soapbox for a minute. Grab your beer and popcorn and bear with me for a short rant....
I think Toyota (and all of the truck manufacturers, for that matter) might be missing the mark...whether because of payoffs from oil companies, or whatever the conspiracy theorists are hyping today, or because of perceptions they have of American truck owners and prospective owners.
A diesel engine naturally has gobs of low end torque (slow combustion compared to gasoline requires a longer relative stroke, which in turn equates to higher torque). Diesel also has more power per volume than gasoline...not to mention it's cheaper to store/transport due to being less volatile than gasoline (and don't even get me started on renewable, natural sources of biodiesels!). Couple a diesel engine with a turbocharger, and it becomes much more responsive to throttle variations, and cleaner burning under load.
But, you don't see any American trucks running anything less than about 5 liter turbodiesels. I suppose that's great if you pull stumps out of the ground or otherwise need 15,000 pounds of towing capacity. But why not produce something like a 2.0 - 2.5 liter turbodiesel truck for the "everyday driver"?
For a point of reference, I recently parted with my 2000 VW TDI shortly after I got my Tacoma (aothough my wife still has her Beetle TDI). I fully understand that there's a significant difference in aerodynamics, drag, weight, etc. However, the final gearing isn't that different. The 2.7 Taco is doing (as I recall) around 2500 RPM to run right around 75 MPH. The VW was doing 2500 RM for 80 MPH. That engine was a 1.9 liter turbo diesel. In stock trim it made 90 HP and 289 Ft-lb....at 1850 RPM. And in the Jetta chassis it got close to 50 MPG. I tweaked it, taking HP up to a little over 150, and toque up to around 360 ft-lb. And it still got 43 MPG (and turned a 15 second quarter mile...while not Earth shattering, it wasn't bad at all for a 40+ MPG car). If Toyota were to stick something like that in the Taco, I don't see why it wouldn't exceed the current 2.7 in MPG (easily high low to mid 30s), and compete with, or even exceed, the 4.0 in torque (and by definition, towing capacity). The footprint of the powerplant would be approximatly the same as the 2.7, the weight would be only slightly more than the 2.7 (and noticably less that the 4.0), so there wouldn't be any major chassis tweaking required.
The other thing most Americans think of when you say "diesel", is WWII destroyer-type plumes of black smoke belching out of the exhaust. But the "German Big 3" (VW/Audi, BMW, and Mercedes) all make turbodiesels that exceed current U.S. tier 2 emissions....so the technology exists. In Europe, somewhere between 40 and 50% of cars, light trucks/SUVs, and vans use turbodiesels. Including Toyota. I honestly think that if they did it right (not like so many of the "compromise" diesel engines we've been forced to tolerate....Jeep Libery CRD, for example), Toyota wouldn't be able to keep Tacos on the lots if they offered a decent turbodiesel option. We don't need 7 liters of snot-slingin', smoke belchin' oil burnin' baddassery to haul our 800 pound truck nutz hanging from our back bumper....just a nice midsize, efficient turbodiesel to get from point A to B, with reasonable towing capacity.
We now return to your regularly scheduled program.