A picture is worth a 1,000 words so take a look at quarterly EDA revenue as compiled by EDAC:

Ron Wilson of EDN has a good summary of Q3 2009 noting that revenue was down 3.3% compared to Q3 2008.
Typical explanations for a decline in EDA revenue include:

Decreasing number of ASIC starts
Increasing number of FPGA starts [...]

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