What size capacitor did you try with your PWM and what frequency did you use?
I was able to go as low as 150uF. As far as the frequency, not really sure, I just varied the duty cycle between 230 and 255. I used DT's interrupt routine as a poor mans HPWM.

Your .58V is so incredibly close to the voltage drop across a standard diode. Perhaps you can experiment with the current through it to get .58V. Use it like a zener diode.
Good idea, didn't even think about that. I'll try that along with the solution below.

I think I may have found a workable solution. It looks like I was just using resistor values too high so I tried the 23ohm and 3ohm combination mentioned in my other post and it works perfectly. The only issue, which I think can be solved quite easily, is that the voltage divider draws about 185mA. Power consumption isn't really a problem because it's not being run from a battery but who wants to waste power?

This may be the bigger issue, I was going to "simply" use one output pin from an 18F4550 to turn the voltage divider on and off but that won't work as I have it now. Obviously disconnecting either the positive or negative leg of the voltage divider will throw off the thermistor value significantly. I was thinking about connecting one end of the voltage divider to one PIC pin and the other side to another PIC pin. I would make the PIC pins inputs when I don't need to force a certain temperature and then making one pin high and the other low when I needed it to work. Do you guys think this will work?