It's not a rumour. Stephen O'Malley has it on his site and Seetickets have aleady listed it too. Tickets are 16 pounds.
http://www.ideologic.org/index2.html
http://www.seetickets.com/see/event.asp?e%7Cartist=SUNN+O%29%29%29+%26+BORIS+PRE SENT+ALTAR
Boredoms date is listed on the site of their booking agency.
http://www.toutpartout.be/boredoms/boredoms.htm
Anywaaaay, thank god/satan both sites don't list it as a "Don't Look Back" thing, like how it was talked about on the SL forums. That would be the worst idea ever: they only have one album anyway and it's barely a year old.