Yahoo-owned Tumblr issued an update for its native iOS app last night, bringing the client to version 3.4.1. But you won't find any new features or UI enhancements in this release—this particular update is all about security.
Apparently the popular micro-blogging service was recently made aware of a major security issue, which in some cases allowed user passwords to be compromised. So it's urging users to download this 'very important' update...