There's so many ways to go about this. As a lowest common denominator, simply having an assets/icon.png would perhaps, unsurprisingly, make the majority of people happy. This could require an asset at 1024x1024 and scale.

As an addition to this, it may be reasonable to allow someone to tweak how scaling is done with a name modifier, e.g. assets/icon_bilinear.png or assets/icon_lanczos3.png. Sometimes a different algorithm is enough to produce suitable results.

With regards to platform specifics, things may get a little crazy when trying to remain platform agnostic. For example, Android defines the mipmap resource directory specifically for defining icons where assets provided may be displayed larger than what is normally provided in a drawable qualifier (mdpi, hdpi, etc) for cases where something like a home screen may display icons larger than what was intended by the system.

With things like this in mind, I don't think it would be unreasonable to allow finite control over defining icon resources to assure the app is presentable for it's intended audience, but this does push against the grain of platform agnostic behavior with gomobile.

With regards to Android, I'm unaware if simply packing the icons in the apk is enough (that'd be great). I suspect that a resources.arsc will need to be generated to identify the drawables for their intended configurations, such as with internal/binres.

Has there been any work done on this? Is it now possible to set the app icon using gomobile?

I'm working on android support as of a couple weeks ago but it's not ready yet.

CL https://golang.org/cl/30019 mentions this issue.

Please add anote to wiki https://github.com/golang/go/wiki/Mobile about this restriction.
Currenlty we are unable to set an icon using gomobile.

What's the status of this addition?

@rucuriousyet this push added the ability to generate resource tables for apks which is then used for the simplest form of icon support, picking up on an assets/icon.png file and packing it into the android apk as an xxxhdpi mipmap. From there, the android system provides scaling for different densities.

It's desirable to support specifying the icon for each density and this pushed removed technical hurdles related.

It's also desirable to support iOS but I don't have the requisite hardware to test an implementation. From what I read a year ago, my impression is iOS support would be comparatively trivial.

Any idea when this may become part of gomobile?

Or has it already?

It has. You'll need to install the latest version of cmd/gomobile to use.