Update: You may want to hold of on this update, as some users are reporting some pretty major issues. The issues only seem to happen if you use the automated Software Update feature. You can download the Combo update directly from Apple and install that with no issues according to this Lifehacker article and their links to another support article. -- Douglas
This one is a BIG one in size, if not in actual changes. You can wait until Software Update notifies you automatically or select Software Update… from your Apple Menu to force a download immediately. Here is a partial list of changes from Apple's Tech Note about the update.
The OS X Lion v10.7.3 Update includes Safari 5.1.3 and fixes that:
- Add Catalan, Croatian, Greek, Hebrew, Romanian, Slovak, Thai, and Ukrainian language support
- Address issues when using smart cards to log into OS X
- Address compatibility issues with Microsoft Windows file sharing
- Address an issue printing Microsoft Word documents that use markup
- Address a graphics performance issue after sleep on some earlier iMacs that use ATI graphics
- Resolve a Wi-Fi connection issue when waking from sleep
- Address an issue that may prevent Safari from opening before joining a wireless network
- Fix a potential issue authenticating to an SMB DFS shareInclude RAW image compatibility for additional digital cameras
A complete list of the more esoteric changes in 10.7.3 can be found on the Apple Web Site.