We had interactive personalization for web-to-print products available for a quite a while. A brand new help post explains how to turn it on on your web-to-print site. Make sure you run 1.9.1 beta 14 or later for it to function correctly. All previous versions have significant faults in that area of code.
And while writing this blog post I found a small hole in the feature. Just letting you know we’ll be patching it soon, in case you encounter it.