The Sony Ericsson Xperia arc has now been rooted by the wizards over at xda-developers. Two of the prominent Xperia X10 devs, Bin4ry and zdzihu, have been brainstorming over the last few days and finally got root earlier today.
To enable root you will have to unlock the bootloader first. You will then need to flash the files pointed to in this thread. This only applies to UK firmware right now, but Bin4ry will download the package for phones using global firmware later today. The Xperia PLAY is expected to follow suit very shortly. Click through for evidence of the Xperia arc being rooted.