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piatkow - 7:40 am on Jun 11, 2006 (gmt 0)
Yahoo and Ask both seem to vary the position far less on small changes. Blue Widgets and Widgets Blue will come out within one or two places of each other while on G the variation can be counted in pages rather than positions.
Your position in the serps will vary according to the search arguement used. Tests on my site have found minor variations in wording with have the postion jumping from no 1 to somewhere so low that I stopped counting.