We (TidePowerd) enrolled in BizSpark towards the end of 2009 -- as we build a .NET-based product, it's been instrumental in building our company. It doesn't cost Microsoft to give us copies of Visual Studio, Windows, etc., and it would've been a major factor in our startup costs if we'd had to pay for a bunch of licenses.
I came across this and i was curious how prevalent this is in the HN community. 45000 is a pretty big number.