update (03 July 2011): According to Livingstone Tourism, the Zambezi River is dropping... which means that rafting is getting better and BETTER! Running rapids 10-25 now!
Wild Horizons, a tour operator situated in Victoria Falls, Zimbabwe, just announced on their facebook page that the Zambezi High Water Rafting season will start approximately on the 11 June 2011. Rafting tours will start from Rapid #15.
update (20 June 2011): Adventure Zone (VicFalls, Zimbabwe) has also opened its Zambezi rafting operations.