I have to chuckle when I see solar recommended as the final solution. No one
seems to consider the amount of energy it would take to manufacture the
panels, to convert the DC to AC, to make the large-diameter/low resistance
conductors necessary to carry such small voltages, and finally, the amount
of land taken out of production because it is in constant shade.

Bob Gardner

The problem with burning hydrogen is that solar panels can't make enough of
it. To replace today's vehicular use of oil, we'd need 230,000 tons of
hydrogen daily.  Solar cells to make that much hydrogen would cover about
20,000 square kilometers.  To get this much power -- 400 gigawatts daily --  
without carbon emissions, the only cost-effective solution is nuclear
plants. And that has its own political and ecological consequences.  See
http://www.world-nuclear.org/opinion/grant.htm

Seth

I think you're mostly right for short term, but long term belongs to
hydrogen.
The problem with solar is storing the energy. Stick a solar panel in your
back yard that converts water to hydrogen, and you can store as much as you
want.

I would like to present to you the (near) future of transportation...

Diesel hybrid cars (full two-mode plug-in hybrids)
Diesel for motorcycles
Diesel (probably 2-stroke) engines for general aviation (including
rotorcraft)
Extensive use of bio-diesel

Hybrid cars (gasoline)are in production
Diesel hybrid trucks and busses are in production
Diesel motorcycles are / will be in production soon
Diesel aviation engines are in production...more are coming

Very soon we will see the first diesel hybrid passenger car in
production

Diesel for aviation or motorcycles...they have been around for some
time. Think of Junkers "Jumo" 205. Decades ago there were several
Diesel aircraft engines built by Guiberson, Packard, Rolls-Royce,
Clerget, Fiat and others. Royal Enfield (India) has had a diesel
motorcycle in production for some time. I believe it is now
discontinued.

Some diesel hybrid car prototypes...

GM's Ope Astra diesel hybrid
Citroen Berlingo diesel hybrid
VW Golf diesel hybrid (so I hear)

Some diesel aviation engine prototypes...

SMA Morane Renault MR 200 www.smaengines.com
Teledyne Continental Motors CSD-283 www.teledyne.com (Nasa GAP)
DeltaHawk V-4 www.deltahawkengines.com
Zoche 01A www.zoche.de
Diesel Air Ltd. DAIR-100 www.dair.co.uk

Some (gasoline) hybrid cars in production...

Toyota's Prius and Lexus
Ford's Escape Hybrid

Some diesel motorcycle prototypes...

Diesel Kawasaki M1030 M1 (KLR 650) F1 Engineering / Hayes Diversified
Technologies (HDT)

For third world countries a diesel motorcycle could be ideal. It is
probably very dependable and durable and relatively easy to fix. It
could be bi-fuel, meaning it could use diesel, Jet A, light heater oil
and most importantly... bio-diesel and even straight vegetable oil
(SVO). It could ne equipped with a small (max 200kg) 2-wheel trailer
and a power take-out (PTO). The PTO would mostly be for a generator
and/or (water) pump.

Picture this. In a third world country, a motorcycle travels 100km @
80kmh with 2 litres vegetable oil that they have made themselves. It
then pumps (with a pump attached at the PTO) from a deep dwell 150
litres of fresh water into a tank that's on the trailer, drives back
and delivers the fresh water to the families in the village. Later at
night, it runs to provide electricity (with a generator attached to the
PTO) for many houses in the village and to recharge the batteries. This
motorcycle and its accessories the families in the village have bought
together, with some help from some organisation like the UN.

Of note: Of course the best way to produce electricity in these often
sunny third world countries is solar panels!!