Apple has just released their latest update to iOS (4.3.5) in iTunes today, following their release of iOS 4.3.4 less than two weeks ago. The firmware update is available immediately, and fixes a security issue with certificate validation. The build number of the new update is 8li.
For Verizon users, a separate iOS 4.2.10 version has also been released, featuring a build number of 8E600.
Below are the direct links to download the new firmware:
You can read more about the security content of the update in this Apple KnowledgeBase article.