But Reuters now says that the S-1 won’t actually come until tomorrow morning. No explanation for the minor delay, although my best guess is that it’s a paperwork issue. Greece-related turmoil might postpone an actual IPO pricing, but not an IPO filing.

Earlier reports suggest that the San Francisco-based company will aim to raise between $1.5 billion and $2 billion at an implied valuation of between $15 billion and $20 billion. The desired valuation, however, is unlikely to be noted in initial regulatory filings. Instead, that usually comes in amended filings once an offering price-range has been determined.

Zynga has raised over $500 million in VC funding, from firms like Foundry Group, Union Square Ventures, Kleiner Perkins, Avalon Ventures, Institutional Venture Partners, Andreessen Horowitz, Tiger Global, Digital Sky Technolgoies, Softbank and Google.

While we wait, it’s worth noting that another Kleiner Perkins portfolio company did file to go public today: Luca Technologies, a Golden, Colo.-based company focused on gasification of in-ground coal, shale and oil reserves. It is looking to raise $125 million, with UBS, Citi and Piper Jaffray serving as co-lead underwriters. In addition to Kleiner Perkins, investors include One Equity Partners, Oxford Biosciences and BASF Venture Capital.