Thanks for the responses. As some of you guessed, I'm trying to avoid a DAC type solution because I want to launch a sine wave into something and use the Arduino's processing power analyzing the returned signal in real time.
I think that with a converter board like the one Grumpy_Mike posted, or one from epboard
I should be able to use the 9833
chip for a pretty neat solution. The 9833 takes a command via SPI that sets the frequency and a clock signal and then proceeds to generate sine waves on its own until you tell it to do otherwise. The ATmega48 has an SPI port and there is supposed to be a way to replicate its internal clock on one of the digital I/O pins so it seems like a good match.
I'll let you know if it works!