Ok, good. Well... The way I do it is as follows. I select the sky using the lasso tool and invert the selection when all of the sky is selected. Then I do the sharpening process for the plane, erasing jaggies, etc. The rest of the background was also sharpened, so when I feel that the background is sharpenend too much (lines on the runway, lamp posts, etc.) I manually erase the rest of the background with the eraser with a bigger brush, roughly around the airplane. You don't have to be super meticulous though, as the background is usually out of focus anyway and USM
doesn't change all that much.
PS, all actions mentioned above are done on the sharpened layer.
[Edited 2008-01-07 00:30:02]