Hey Jamie,
I've had a couple of folks in the forums asking for the ability to disable clamav. Seems like maybe this is a reasonable option for the "Spam and Virus Scanning" module configuration page (though I guess it needs to reflect through to the per-domain spam delivery configuration page).
I suspect that this might be because clamav is still running into problems on some systems and is preventing mail delivery (there was an issue like that in the forums a couple of days ago--but it it was for GPL and there's not much we can do in that space until Spam/AV features find their way into GPL), but, so far, I'm not getting good answers from folks on why they want to disable it. Nonetheless, there are good reasons for not wanting virus scanning to happen--maybe they run an incoming virus/spam proxy box in front of all of their servers, and don't need it on a per-system basis.