Hello. I've just finished up an Arduino based mod to my espresso machine, a second revision to an originally PIC based one. Here is a link to the blog entry:
There are basically no technical details on the blog, but I will make available all the code. Also not included in the blog is a demo of the Processing based back end that allows control and tuning of the machine from a PC. A screen shot of that, it's code, plus a stripped down version of the coffee controller arduino code are available in the playground at: Arduino Playground - BarebonesPIDForEspresso
I like your work. If I ever manage to get myself a decent espresso machine, I'm going to try a PID controller mod on it.
One thing that interests me is whether you could "autotune" your PID controller using a step input (ie. turn the heater full-on), measuring the response (the lag Td and maximum slope of the resulting S-curve) and applying the method as per part D at the end of this.