We at Showcase find Adobe InDesign one of the most useful pieces of software to design Showcase slides with - it's got a good balance of typesetting, and image-based capabilities, including robust vector support.
If you're thinking of using InDesign to create your Showcase slides, we've made it one step easier for you, with an IDML template.
This template is at 2048 x 1536 pixels, the default Showcase size, which suits all manner of devices and Retina iPads in particular. (For more on the different sizes and ratios that Showcase supports, check out our design specs doc.)
We've put the "Showcase controls" - the circular buttons in the corners - on their own, transparent, non-printing master layer. So you can design around these controls and then export a clean image when done!
To grab the template, click here for a direct download.