shutterfly studio has something like this. You have to download
Shutterfly studio software first then once it is set up
you select your folder of picture then select your size layout (8x10 or 8x8), then drag and drop pictures into the collage of your choice.
You can preview it then order it from Shutterfly right in the software.
You can save the flat JPEG collage but not a layered version, that might be a drawback if you want to use different background.
It is very similar to Picasa software but I haven't had as many problems with it as I did with Picasa. Plus, ordering is much easier.
Probably not what you are looking for but thought I would try anyway