What I am looking for is a Camera (with phone) that can be installed in
a vehicle, with motion detection, such that when a person enters the
vehicle, a picture is taken by the camera and sent via GSM/cell network
directly to another camera phone on the same network. If motion
continues to be detected, the camera will continue to send pictures once
every 30 seconds (or some other settable period).

Preferably, the camera in the vehicle will be separate and will
connect to the phone in the vehicle, rather than be a camera built
into a phone.

I understand that this will require two cell phone plans/phone number
subscriptions.

Is there a solution out there, or is there a developer who can provide
a fixed price quote for providing a working solution?

You will need the following:

1 - A cheap laptop or PDA> I can further qualify this
2 - A camera for the laptop  >$50-$100   or a wireless camera for the laptop $250?
3 - A wireless PCMCIA card > $150 Verizon Wireless > $60 a month there after
5 - A power converter from DC to AC to support the laptop/PDA > $60

You will NOT be able avoid using a laptop/PDA.  <<  I can also qualify this.

Cheers

IMHO the best way to go is an Windows Mobile based Smartphone (for
example the Orange SPV C500 in Europe, aka Typhoon).

Applications for these devices are quite simple to develop. It is
possible to develop an application that captures video frames and
sends captures pictures via MMS or E-mail to another phone or some
E-mail account. Motion detection is pretty simple to implement by
comparing one frame to another and count the changed pixels.

What you need is:
- Cell phone
- Development Environment (Microsoft eMBedded Basic or eMbedded C++)
- SDK (software development kit) for the phone
- Some guy to implement it

Also don't forget that the cellphone in the car should get power from
the car's battery because the life time of the battery of the
cellphone is not very long if the camera is always on.