I'm close to wrapping up Acorn 4.5.2, which has a handful of fixes, as well as two minor features.
The first feature is support for @3x images with Smart Layer Export. If you have a @1x layer you want to be automatically scaled for the other sizes, you can set the name of your layer to "icon.png, email@example.com, firstname.lastname@example.org", and Acorn will automatically scale up the layer to match the right sizes. If you name your layer "email@example.com, firstname.lastname@example.org, icon.png", Acorn will scale down the images. This works pretty well with bitmap layers, but it really shines with shape layers. And of course you can mix and match different types of layers inside a group layer.
And new in 4.5.2, Acorn will auto-detect if there are no transparent pixels in your image and no longer include an alpha channel when you save or export your image as PNG. This should decrease the size of your PNG files even more.
As always, you can grab a preview Acorn 4.5.2 from the latest builds page.