If we take a literal defnition of contact to be 'the soft, steady connection between the rider's hand and the horse's mouth', then this would be difficult without a bit. But if we are looking more at the overall back-to-front connection then maybe this is possible?
The issue of whether the FEI should allow riders to compete in bitless bridles was discussed at the 2011 Global Dressage Forum and you can read the report by Eurodressage here: Bitless or Not, it's about Having the Choice.
The Ridden Horse Behaviour project is focussed on assessing and describing the observable behaviour of ridden horses rather than training methods, so we are sitting on the fence with this one! Do feel free to post your own opinions below though, but watch the video first!
(It is not possible to embed this video, so please click on the link)
Uta Gräf riding bitless: video