Yes, and it's done from the Contacts app itself. Open the contact you wish to silence with a silent ringtone. Note that you must already have a silent ringtone downloaded onto your iPhone.
Select "Edit" for the contact and scroll down to see the notifications settings for that phone. Tap the current ringtone in the contact and choose the silent one you have installed.
Remember to tap "Save" on each screen until the contact is closed.
I go a level further than muting individual contacts. My default ringtone is 30 seconds of silence; everybody is muted. When I add a new contact I assign an audible ringtone to that contact. Usually it's Scotland the Brave. Bagpipes will get ANYbody's attention
. I do something similar with text alerts. While my default for everybody is an old BlackBerry tone, work contacts get the Nextel Chirp. Annoying enough to get my attention even in the middle of the night. Family gets a different BlackBerry text alert - louder than the default notification but not as jarring as that Nextel chirp. Wife gets her own alert tone, barely noticeable and easy to "miss." Oops