Last week the developers of Zen Cart released a new version 1.5.6B.
I have been watching all the bug reports and have not felt comfortable upgrading people to 1.5.6 until this version came out, which would include all the reported bug fixes and security fixes found in 1.5.6 and 1.5.6a.
Check our upgrades section if you are interested in getting the latest, most secure version of Zen Cart to run your store with.