No, they'd just require the private company to disclose the same sort of quarterly information that a public company does.
You're clearly alluding to Facebook being "forced" to go public, but they too could remain private with 501 investors. But once they're revealing their numbers, they figured they might as well jump in with both feet and do the IPO.
Typically when a company goes private, it buys back the shares from the current shareholders. The number of shareholders would then drop back under 500.
It's actually Float Shares * Market Price.
This doesn't make much of a difference in Apple's case, because most of their market share is floating on the public market. Quick calculations:
932.37M Shares Outstanding x ~$600/share = $559.4B Mkt Cap
931.79M Float Shares x ~$600/share = $559.0B Price to go Private
$400mm, in this case, doesn't make much of a difference, but if you talk about this for other companies in the future, keep it in mind.
Source: http://finance.yahoo.com/q/ks?s=AAPL+Key+Statistics