On November 22nd Arri announced the SUP 5.0 firmware update for the Arri Alexa. This update, together with the purchase of a new license, will allow the trendy camera to work at up to 120fps in ProRes 422 HQ (10b) 16:9 codec. This update (which among other things will also allow recording in ProRes 444 at up to 60fps) is free of charge, but the High Speed license will cost extra.
You can see the rest of the features of the new version and the press release at Creative Cow