With the 18 on the 2010, the fender comes into the rim more, therefor looks slammed..........
I'd like to merge this comment with JC's below:
I measured from the center of the front edge of the fender to the tire. The wife's 09 SG is 2 1/4". My 10 SESG is 1 1/2".....a difference of 3/4". 
Here is what I show:
Stock 2009 Street Glide Tire (front): 130/80B17 65H = Rotational Diameter of 25.18"
Stock 2010 Street Glide Tire (front): 130/70B18 63H = Rotational Diameter of 25.16"
Feel free to check my math, but if there is 3/4" difference in fender clearence (per JC's post above) then I don't see how a 21" will fit on the stock setup without some mods to the front fender. I <casually> checked some some sizes of front tires available for the SESG and there doesn't seem to be a "low profile" tire available that will allow a final rotational diameter within .50" of the stock rotational size. That being said, I'm not sure about the method of enlarging the holes in the fender would allaow for the fitment of a 21" front (with available tires).
Again ... check my math but I'm smelling a different fender for a 21" front wheel. The only variable would be the mould of the front fender on the 2010 SESG. Meaning ... it could have the same internal clearance as the 2009 SG, but has been formed such that the very front portion of the fender is shaped so as to appear more "slammed".
Dunno ... just offering stuff up for consideration.
