The first bundle (and possibly second) was bootstrapped, they took funding after the idea was shown to work. That's all I know, not sure why they have funding.
Thanks for the extra info. Seems to me like this is the perfect company that can eschew outside investment; I assume they curate these bundles with no upfront payment, and then everyone gets a cut of digital sales. Not sure where this $4m could be going unless they have big plans beyond the current the current model.
Braid can be bought directly with Paypal, Amazon Payments, and Google Checkout as payment options. Corner of the box says "Humble Store". What they do with that is a unknown, but I find it interesting.
That reminds me: I noticed that humblebundle.com is apparently involved in the purchase transactions for the recently released 'English Country Tune'. So maybe there's more afoot than just the bundles themselves:
edit: I was wrong, not just YC. They have $4.7m from Sequoia[1] too.
[1] http://finance.fortune.cnn.com/2011/04/22/sequoia-capital-ba...