Cool project...
I recently saw a project like this in Make #11. The project went something like this;
PHP web page scraper -> Latronix serial to ethernet -> Arduino -> Analog Meter
You can use Pulse Width Modulation (PWM) to sort of "average" out the on/off pulses inherent in digital signals to output to your analog device. Check out; MAKE 11: Alt Vehicles - Make: DIY Projects and Ideas for Makers for a few ideas on code.