The units will be decimal degrees.
What you might find useful is to buffer around your center-point.
Try something like this:
That will use Meters (WebMercator's units). Note that your output may not be what you expect as the input or output approaches the poles. Also, there's no guarantee how many points the output polygon will have in its boundary.
-(AGSPolygon*) circleWithCenter:(AGSPoint*)point radius:(double)radiusInMiles
AGSGeometryEngine *ge = [AGSGeometryEngine defaultGeometryEngine];
// Get a point in a spatial reference whose unit is Meters.
AGSPoint *pointToBufferAround = (AGSPoint *)[ge projectGeometry:point toSpatialReference:[AGSSpatialReference webMercatorSpatialReference]];
// Get a "circle" around it, using the unit Meters (1 mile ~= 1609.344 meters).
AGSPolygon *circle = [ge bufferGeometry:pointToBufferAround byDistance:radiusInMiles*1609.344];
// Project the output back to the projection of the input point (which is presumably what you're after).
circle = (AGSPolygon *)[ge projectGeometry:circle toSpatialReference:point.spatialReference];