A few weeks ago, my wife's Sienna's battery gave up the ghost on a really
cold day (not surprising since it was almost 4 years old).  I was tempted to
find out if the Prius could jump it, but I figured since it was not intended
to crank a starter motor, I reasoned that the current capacity ("cold
cranking amps") probably wouldn't be high enough for an effective jump
start.  I definitely didn't want to have two dead cars that day.
Fortunately, I have a Xantrex backup power supply (essentially a big
lead-acid battery packaged with an inverter on wheels with a handle) which
worked perfectly on the first try.

Nevertheless, I am also interested in the answer to this question.  Mine is
a 2006 as well.

- Doug

A Prius' 12v battery shouldn't be jumpstarting another car's battery.
(Too easy to blow the fuse on your donor Prius.)

The Prius' 12v battery is smaller than an average car's 12v battery,
because it never has to start an engine. So, if you do a traditional
jumpstart/boost, you can overpower the Prius' 12v battery, and
potentially blow the 100 amp fuse that charges it (from the hybrid
battery).

The preferred method of jumpstarting another vehicle, using a Prius
is:

Turn off both cars.

Take the keys of the car to be jumped, so that the owner doesn't try
to start their car on your donor Prius' battery.

Connect up the jumper cables per the owner's manual of the car that
needs to be jumped. It's probably best to go direct with the Prius'
12v battery terminals, found in the trunk/boot, but if you have a
NHW20 (2004 and newer) Prius you can probably use the under-hood jump
points in the fuse box.  Watch that polarity!

Turn on the Prius to READY, so that the gasoline engine can charge the
hybrid battery, the hybrid battery can charge the 12v battery, and the
12v battery can charge the dead car's battery. Leave like this for at
least 15 minutes to recharge the dead car's battery.

Turn off the Prius, and disconnect all the jumper cables.

Try starting the dead car using its own somewhat charged battery.

Drive this car around for a while to let it recharge its own battery
more.

This'll only work if the dead car's battery is merely too weak to
start the car. If it has any damage or an internal short, it won't
work (and could be dangerous to charge it).

The real preferred method, other than having an auto club come out, is
to have a portable jumpstart kit for your car. Less than US$50, you
can get a jumpstart kit that also has other neat stuff in it, like a
compressor for tire inflation, or hazard lights. Something worth
having in your car toolkit.

Jump start procedure if needing to jump your own Prius is in the "in
case of emergency" section of the owner's manual.

In a traditional car, you'd have to drive around for a while after a
jumpstart to get your 12v recharged, because the alternator is run off
of the gasoline engine. In a Prius, which doesn't have an alternator,
you just have to leave the car in READY to charge the 12v battery off
of the big hybrid traction battery. (But, if your car is on, you might
as well drive it somewhere...) A good 30min drive somewhere should do
the trick.

(Don't forget that US 2001-2005 (and maybe 2006 if you have the right
paperwork) Prius come with 3 years/36,000 miles of Toyota Roadside
Assistance, which includes jumpstarts.  If you have the Toyota Extra
Mile package (Southeast or Gulf States Toyota region) or if you bought
the Prepaid Maintenance package, you may also have the Toyota Roadside
Assistance.)

Last year, just some month after receiving a new Prius, I left it for
some days parked without actioning the automatic door closing system.
The light on the trunk was switched on and the 12 V battery went out.
I had  problems in restarting the car, but then I realized it was a
stupid think: the secondary battery is needed only to start the
computer, so also a normal torch with 12 V DC is sufficient. The
motion battery is very well protected and it is improbably fully
discharged.
In the engine bay of the car, in the fuzes box, there is a +12V DC
with a red cover: you can use this and the vehicle ground, applying
12V DC to restart the computer (less than 1 Amp. is required).
For this reason you cannot use the Prius to start another car, but, of
course, you can use another car to Jump Start the Prius.