I got a Spark a few days ago and I love it -- I've been flying it several times a day since I got it up and running. Today I noticed that it yaws (slowly turns in one direction) when hovering, and it seems to fly slightly slanted when I'm telling it to fly straight (way more noticeable in sport mode). I've re-calibrated the IMU, remote control, and the compass. Has anyone else experienced this? If so, what did you do to fix it?

I would try a stick calibration   

I would try a stick calibration   

the ac naturally dips in the direction of travel. The may be the result of being in ground effect. Try the ac higher than body height and see if it still yaws. If so, then try an control calibration. Keep in mind even in hover with good gps, the ac(phantom) will show some mild drift.