16 bit printing is finally possible. It wasn’t before? That’s right.
Only Photoshop CS4 / Lightroom 2 actually deliver all 16 bits to the printer.
Only OSX 10.5 allows this; not Windows yet.
And only the latest updated drivers.
What improvements can you expect to see? Smoother gradations. Slightly more precise edge rendition.  As printer dot structures get finer and more precise, expect to see more improvements. Higher quality digital capture (higher bit, lower noise, greater resolution) will also improve output.
Find CS4 and Lightroom here.
Find Epson drivers here.
Learn more in my Fine Digital Print workshops.