Sharpening itself will not present any difficulties, but I know Photoshop struggled on the iMac retina when it was first released. I'm pretty sure all the issues have been resolved, though.
It's likely that the image will look sharper on a retina screen than a non-retina. So in terms of how much to sharpen, if you're still using the same camera as before you should continue to use the same sharpening settings you were using previously.
In photoshop, use "Filter > Sharpen > Unsharp Mask..." with an appropriate mask to avoid the halos MCAsan mentions above.
Hope that helps.