I'm also curious. From the digging I've done. A lot of the numbers can be seen on CrunchBase.com from the TechCrunch guys. They will rarely share the percentage equity given away though. I'm guessing this is something founders want to keep a secret!
A lot of the time this is done to keep secret any strategic investors in a company.
Some very large companies will plop some money down just for the sake of starting a relationship with a company so that they could monitor that company to make an acquisition offer when the time is right. Sometimes the founders want to keep certain investors secret because it may scare off investments or acquisition offers from the competitors of the strategic investor. If you are a founder you should definitely deny board observer rights and financial observation rights to such investors. They can work against you as a de facto poison pill.
Another option is that there are "gatekeeper" investors involved that used to work very high up in some large public tech company. They discover a new company that may be of interest to their former employer. They make a nominal investment in the company and then make a ton of money back by simply making the right introductions and pushing the deal through. While this conflict of interest may be known to many of those involved, they may not want their investments to be openly public since it would shed light on the conflict of interest.
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