Businessweek has stated that Zynga is planning to raise about $900M in the IPO with shares to price between $8 to $10. 10% or less of outstanding shares are expected to be sold. This would give it a valuation of about $10B. Compare that to the two current leaders in the space: Activision Blizzard (ATVI) with a market cap of $14.2B and Electronic Arts (ERTS) with $7.69B.
Sharepost implied valuation currently stands at $12.9B.