Doesn't look like there's an opt-out, the McAfee 12-month subscription is included in the price that was paid.
http://malwaretips.com/Thread-3link9-s-XPS-8700
http://www.dell.com/uk/p/xps-8700/pd (UK site)
Yeah Dell gave me not 12 months (1 year) but they gave me 3 years subscription plus 32 months of live safe. So I'm keeping it, I'm liking it so far.
I used to be a long-term McAfee user of 8 years before I tried other software so its not new to me.