Within the last few minutes, iOS 4.0.2 for iPhone 3G, 3GS, 4, and iPod touch 2nd and 3rd generation (late 2009 models with 32 or 64 GB) has become available for download. In addition, the iPad has received an update to iOS 3.2.2.
The updates fix a security vulnerability that was associated with viewing malicious PDF files. It doesn't appear that there are any fixes to any other issues (i.e., the proximity sensor glitch), but the fix to the PDF exploit insures that the iOS platform remains secure.
For a security fix, the update is huge -- the iPhone update is 580MB in size and the iPad update is 457MB. For a look at more detailed information about what the updates patch, check out the iOS 4.0.2 Update description and the iOS 3.2.2 Update description on Apple's website.