Thanks to the hardworking devs over at the XDA-Developers forum, the Xperia arc(sic) has been rooted. The pair responsible for the breakthrough, Bin4ry and zdzihu, released the how-to info early this morning (or late last night for you insomniacs). It seems like a pretty simple process: Unlock the bootloader, download Bin4ry's file, and flash it with fastboot.
This currently only applies to the UK version of the device, but it is expected to be ready for phones running the global firmware sometime tonight. Head over to the XDA thread for more info and download links.