Citing an increasingly challenging economic and competitive environment, Comcast trimmed its financial guidance.Revenue Generating Units (RGUs) are now expected to increase by approximately 6 million to 57 million, versus previous guidance of approximately 6.5 million additions. This represents a significant increase compared to the 5 million RGUs added in 2006. Comcast said this RGU growth is expected to contribute to cable revenue growth for 2007 of approximately 11%, compared to previous guidance of at least 12%, cable operating cash flow growth...