iOS 5's on-device delta updates can't come soon enough. Just a week and a half after releasing iOS 4.3.4, Apple has dropped iOS 4.3.5. Apple says that the new update fixes a security vulnerability with certificate validation.
The recently released iOS 4.3.4 focused on patching PDF vulnerabilities that some folks were using to jailbreak their devices, but could be exploited for more nefarious means, too.
As is the iOS update tradition of late, this patch comes in two different flavors: The 4.3.5 update applies to the iPad and iPad 2, the third- and fourth-generation iPod touch, the iPhone 3GS, and the iPhone 4 (GSM model); if you're using a CDMA (Verizon) iPhone 4, you instead will upgrade to 4.2.10.
To get the latest iOS updates, plug your device into iTunes and follow the prompts.