This is a slightly taboo subject because with many product launches there is a launch competition not only for sales, but also for pre-launch opt-ins.
If you are offering an incentive for people to just opt-in, there is a high possibility that that lead might be worth less than a lead that has viewed some launch material, and decided that they are interested in the product enough to hand over an email address.
Something else to avoid is any mention that you might benefit financially from just an opt-in - the problem there is disclosure, and as more and more launches include a pre-launch opt-in competition with prizes, especially in the internet marketing niche, affiliate program managers are going to specifically have to allow, and encourage disclosure.
Bonus Delivery For Opt-Ins