Google, along with a number of other high profile websites, have been migrating their sites over to use SSL encryption for all website pages rather than the historical usage for register / login / account and shopping cart pages and there are rumours flying around that Google may add whether a site is encrypted using a SSL, as one of their many ranking criteria.
Historically, there were a few reasons to choose to not have your website protected with an SSL which included the cost of the SSL itself, the fact that until fairly recently a dedicated IP address was also required in order to have an SSL (which with the shortage of IPv4 addresses and the additional cost is a double reason) and finally the response speeds of pages encrypted under SSL was slower than unencrypted pages. All of these are not really an issue any longer as the cost of a basic SSL certificate is minimal per year, your web hosting company should support SNI when a dedicated IP address is not required (although there are some other benefits still in having a dedicated IP address) and the speed differential these days is small.
The rumours started about a month ago when Matt Cutts, who is the head of Google’s web spam team, said he would like to see Google make SSL site encryption a ranking factor in Google’s algorithm. Now at the moment this is reported as a personal opinion and he did not say that it is or it will be part of the ranking algorithm but as Matt Cutts is a senior Google search engineer and the senior “face” of Google for all SEO / web spam matters – this statement has been taken seriously.
It doesn’t look like anything new has really happened with this yet, and if it is included in the future we would not expect it to have a big effect as that could make it more easily exploitable (although Google are getting so much better at determining abuses of their ranking signals !) but for the minimal cost of adding a SSL, why not do so and we have long strongly recommended it for E-Commerce stores and any other stores that have registration / login / account management or any form of shopping cart pages regardless of whether it is strictly needed for the payment gateway.