Hello,
you should add a R-2R layer on the Arduino board to use Arduino as a programmable function generator.
See for example :
http://www.hit.bme.hu/~papay/sci/DDS/start.htm
http://www.myplace.nu/avr/minidds/index.htm
Regards
This sounds like it would be a better fit for either a shield or a portion of a dedicated daughter/application specific board that also had (for example) a reconstruction filter and an output buffer.
Hey
ladyada has a blog post here describing a one-chip synthesizer by Linear technologies that puts out 1hz to 68 Mhz...
and it's SPI too.
D
Here's more on signal generators
http://www.edn-europe.com/pcsserialportcontrolsprogrammablesinewavegenerator+article+228+Europe.html
You are kindly invited to visit my blog 
which includes a detailed description of a simple and pretty efficient DDS
http://didier.longueville.free.fr/arduinoos/?p=240