hey it is quite the party in here!
But my first thought had been "I'll bet I can get an Atmega8 to serve as a cheap, precise arbitrary square wave generator."
Instead of direct synthesis, what about an XTAL osc, fed into a high-frequency divider controlled by the Arduino? It's basically what they would call a VFO or Variable Frequency Oscillator' in ham radio. That way the Arudino's pins could be taken up by I/o for displays and button for setting the frequency.