I've just downloaded and installed Android 2.3.4 Gingerbread Update and other software enhancements that go with it on the Sony Ericsson Xperia Arc.
Some improvements that I immediately noticed on the 2.3.4 upgraded Arc include, faster boot time, new 3D Sweep Panorama capture, new 3D gallery, refreshed TimeScape user interface, new apps discovery and sharing feature, and slightly longer uptime.
If you own an unrooted Sony Ericsson Xperia Arc, get this update by connecting your phone to a PC and running Sony Ericsson PC Companion. Just wait for the software update notification to pop up. Download the upgrade files then install.
You may also get the firmware update Over-The-Air (OTA). Go to Settings, About the Phone, Software Updates, Check Now. If you don't get the OTA notification, just connect your phone to a computer and run Sony Ericsson PC Companion.