Sorry that's not the case. In fact the card inside the box doesn't mention 'demo' anywhere, only that a "free instant download" can be yours.
If a demo for the upcoming Splinter Cell: Conviction was on the disc, or available via a code, Ubisoft would have been shouting that from the rafters this whole time. Whatever this freebie download is it could just be an 'exclusive' wallpaper or teaser trailer.
The demo rumour got traction from, who spotted the ruckus on .