EDIT: For some reason this doesn't look sharp to me. I just opened the jpeg in preview again to check and it looks fine, so I don't understand what's going on.
Three things are going on here.
1. Lighting is everything. I can tell that you used flash, but it's still a tad underexposed. Was this a popup flash that you used or off camera and bounced? The lighting can account for how easy it is for the AF sensor to find its target.
2. Whether or not it is sharp, doesn't account for the focus being a bit off. Looks like it's front focusing just a hair, thus making the focus soft.
3. Sharpening in general is a tricky thing. How did you sharpen the image (with what and with what kind of pass/filter etc?) to begin with, and where is that image being hosted? You have to remember that most websites or hosts use their own compression schemes, so what might have started out as a sharp image directly from Preview or a photo editor, is easily ruined by such things. iWeb is a good example of such a thing. Apple's compression scheme with that host is fairly heavy handed and I'd never use it as such.
I like ImageShack for hosting single random images. They don't really use much in the way of compression. Same with atpic.com. Photobucket is alright too, but not as good as the other two. Then you have to deal with the website on which you're posting the picture. That too, will usually have software which compresses images in order to save bandwidth. Not sure about this forum, so maybe one of the mods/admins will be able to answer that one.
Basically then, when you're saving your jpg from whatever program you're using, it's usually good to use a bit of extra sharpening. This is especially true for RAW images. For those, you want to apply sharpening while editing, as well as output sharpening. Two totally separate things.
If you knew all this already, sorry for the rant!
Doug