Disney will announce a price increase before they kick it into effect. You can always buy then.
There's usually little to no advance notice of a Disney ticket price increase.
I think historically over the last few years the prices have gone up around August 1, so it's entirely possible they could go up twice between now and December 2010.
Only Mickey knows when the prices will be increased. It's true that 3 of the last 4 price increases have occured in August. Is that a trend that will continue? It's anyone's guess. (In the larger context, only 3 of the last 29 price increases have occured in August.)
Dates of historical ticket price increases:
February 1990
February 1991
June 1992
May 1993
March 1994 (park tickets)
November 1994 (annual passes)
February 1995 (park tickets)
December 1995 (annual passes)
February 1996 (park tickets)
October 1996 (annual passes)
March 1997 (park tickets)
September 1997 (annual passes)
April 1998 (park tickets)
September 1998 (annual passes)
May 1999 (park tickets)
October 1999 (annual passes)
January 2000
January 2001
September 2002
June 2003
February 2004 (ultimate park hoppers)
March 2004 (annual passes)
October 2004 (single-day tickets)
January 2005
January and August 2006 (all ticket prices raised both times)
February 2007 (water parks)
August 2007 (all other tickets)
August 2008