90% of the responsive design is the menu, which we have a basic solution for and are rolling into powerstore designs for an update that should be available soon.
Then we will probably create a tutorial for how to manually insert the responsive aspects into existing sites for people that have already launched powerstore sites.
There will always be at least some custom css necessary depending on the amount of custom pages you have added to your store which will be that last 10%. We should have the responsive designs ready soon. As soon as you see the powerstore update released, give me a call and we can talk about implementing it into your existing store.