My first blush threshold issues are
(a) what a Duemilanove can and can not do?
Well it can't nit your socks. 
This is just too general a question to have a good answer. The Arduino is a small general purpose micro controller and can run programs up to it's speed/memory limitations.
(b) how many SSRs can be controlled by one Duemilanove?
Up to the number of output pins available. Most SSR inputs will work with about 10ma of +5v signal, so that is compatible with the chip total 200ma spread over it's 19 possible output pins.
(c) hot to make it work and play well with a TC?
TC have very low millivolt levels and are non-linear. So they require external circuitry to be able to interface with an Arduino. These can be costly depending on how many TC points you require. A better method or temperature sensor might be more coming if you could define the number of temperature points required and their min/max operating range.
(d) I want to use a PC to drive everything and need to understand the interface.
A Arduino board communicates via USB connection as a simple comm port to the PC. So any PC based application or programming language that can access comm ports, can be made to work with an Arduino.
(e) Can the Duemilanove take the TC feed back and display that on my PC monitor? If not how do I get that date to display in real time?
Yes, with the proper program in the Arduino it can send any information it has to a PC.
(f) I will want to use PWM technology to ramp the DC down to my my peltier's over some period bringing them to idle before turning them off so that all the residual thermal energy from the hot side does not migrate into the fermentor. Can a Duemilanove do that? how?
The Arduino has analogWrite commands that output a PWM signal. This is a simple TTL output signal and would have to work with an external switching transistor to be able to control a high current device like a peltier device. This is the same method that a Arduino can be used to PWM control DC motors and high power lights.
(g) Can the Duemilanove tell time? more to the point: Can it control a process based on time elapsed?
Yes, but this is best done with a small module connected to the Arduino called a real time clock. However seeing that the application has a full time PC attached to it, it is possible with proper programming on the PC and Arduino for the Arduino to get time/date information from the PC and use it in it's program decisions.
(h) Can the Duemilanove track and log and display to a PC monitor, in real time, the things it's done over what time periods?
Well the Arduino does have limited memory resources, so logging of sensor data over time is best done on the PC side as long as there is already going to be a full time connection. The rest as said before is simply the Arduino reading it's inputs and passing the values to the PC for display/logging, a common application. The run duration would be the size of file storage on the PC, so almost unlimited for practical purposes.
Lefty
And of course this recently posted project may be of interest to you. Always better to steal solutions then to start from scratch.