Is it possible to open and edit Photoshop .psd files with GIMP that are using smart objects? (I'm asking because I would like to add a Android app screenshot into a "hand holding Android device" mockup photo)
GIMP does not (yet) support smart objects like in Photoshop (there's an open feature request about it), so the only way they could be imported would be by converting them into something else that GIMP does handle (most likely, a simple "dumb" layer, or a set of layers). I don't know (and have no way of testing) whether GIMP's PSD importer is smart enough to do that, but I suspect it isn't.
Your best option, if possible, is probably to make the edit directly in Photoshop. If you absolutely must use GIMP for this, see if you can get the author of the PSD file to export it into some standard, non-proprietary format like PNG or TIFF — you'll lose most of the information about things like layers and smart objects, but at least you should be able to open and edit the image without fear of the importer messing it up. Also, if the image contains a lot of vector art or things like replicated smart objects, consider exporting it to a vector format like EPS, PDF or SVG, and editing it in a vector editor such as Inkscape instead of GIMP.
In any case, IMO, the PSD import feature in GIMP should generally be used only as a last resort, when all you have is a .psd file and no copy of Photoshop to edit it with. It's better than nothing, but far from perfect, and always likely to lag somewhat behind the latest new features and quirks introduced by Adobe. Apparently, keeping the GIMP PSD import up to date with new Photoshop features is not currently a high priority for the GIMP developers anyway (although, as usual with open source software, volunteer contributions are always appreciated).