Ok, wait, this one I'm seriously researching for a new business idea I'll tell you about later. But I need to figure out what technologies are out there, their costs, and their pros and cons... then make a widget that allows people to customize their item to some extent.

There is a lot of new technology out there that can apply graphic images to 3d metal and plastic objects.  So far I've seen:

1.) old fashioned engraving (slow and expensive, questionable scalability)
2.) laser etching for logos and line art (allows for digital fidelity, but looks kinda cheap)
3.) hydro-dipping - http://en.wikipedia.org/wiki/Hydrographics_(printing)
4.) epoxy paint/'Duracoat'/baked

...how do they do this:
http://www.cafepress.com/make/photo-on-metal/
...and can they do that on something not perfectly flat?

Not seen that before, but there is an on-page clue: "both inks and coating are archival, guaranteed not to fade for 75 years. ".  So printing, which probably means it needs to be dead flat.

Maybe this? - http://www.directcolorsystems.com/printer-applications/anodized-printing.php

Here's another machine that maybe it - http://kgtprinter.en.alibaba.com/product/518306120-213141019/Thick_metal_sheet_printer_printing_machine_Aluminum_printer_printing_machine_can_print_on_all_kinds_of_metal_materials.html

http://i.materialise.com/blog/entry/i-materialise-launches-stainless-steel-3d-printing

>costs, and their pros and cons

There was a local laser-cutting shop that had to relocate to another nearby area just to get a cheaper electric rate. Of all the above, I think laser is likely to have the most expensive operating costs.