Hey Jeff,
As of right this moment, we don't have the subscription service built in... but there is a client that we expect to go live with in the next couple of months that will be doing a very in-depth subscription service, so obviously the those features will be built in over the corse of the next month or two. As for google checkout, it looks like that will be added in the next week or so. In fact if you needed it sooner, I could probably have it up and running by the end of the week.
To me it sounds like the big deal is the subscription service. You may want to contact ten24web (the primary software company supporting slatwall) and ask about getting a solid timeline on the subscription service and to make sure that all the features you need get implemented in our original implementation of it. Because we are a free and open source project your help in directing the development of our subscription features will not only be very helpful to us, but also ensure that you get exactly what you need for your client.
I hope that helps answer your questions.