looks to be a piece of dirt. be careful with it, try gentle rubber 'rocket' air blower. don't want to scratch the sensor or filter.
I end up having to get my sensors cleaned every couple months, it seems like.
Earlier, I tried the sensor swiper stuff, ended up scratching a sensor and had to replace the entire sensor assembly. arggh.
Yeah, I clean my sensor just about 4 times a year. But I don't know if that was on the sensor or not because I took some shots of the full moon last night and there was no spot there.
xander, your sensor spot probably got 'cleaned' off by the in-camera sensor-vibrate procedure.
it had to have been a big something, to show up at f/5.6.