Implementing I2C to analog
Consider the Microchip TC1321 it's an I2C digital to analog converter (DAC) with 10 bits of resolution (4.88mV/step), available in SOIC8 (you'll need an easy to solder adapter board [eg sparkfun's or futurlec's) and operates from 5 volts. If you're OK with only 8 bits (19.5mV/step) consider the TC1320.

Both chips are available from Mouser or Microchip Direct for around $2.

If you can use SPI instead of I2C, your options are easier, the MCP4821 and MCP4822 are single and dual SPI DAC chips that come in nice PDIP8 packages. Again they cost about $2 from the usual places (mouser, digikey, microchip direct, farnell).