Yes.
http://dictionary.reference.com/search?q=instrument
in·stru·ment 
(
P )
Pronunciation Key (

n

str

-m

nt)
n. - A means by which something is done; an agency.
- One used by another to accomplish a purpose; a dupe.
- An implement used to facilitate work. See Synonyms at tool.
- A device for recording, measuring, or controlling, especially such a device functioning as part of a control system.
- Music. A device for playing or producing music: a keyboard instrument.
- A legal document, such as a deed, will, mortgage, or insurance policy.
tr.v. in·stru·ment·ed, in·stru·ment·ing, in·stru·ments (-m

nt

)
- To provide or equip with instruments.
- Music. To compose or arrange for performance.
- To address a legal document to.
Yup, there are a lot of definitions of an instrument. A dentist checks your teeth with an instrument. Similarly, Eminem makes me feel like my teeth are being drilled.
Joel's right. It's kinda pointless to debate whether or not they're instruments. The issue is whether they're musical/editing/whatever in nature.
(BTW, I think scratching can sound cool sometimes, and all that, but I still see it as much a function of editing, like a fader, reverb, or pan).