@Jeffro, you CAN use Akismet for free, I don't argue with that. Even with a "big" site, but should you? The price tag akismet puts on it's service is pretty clear, you've got to pay. They don't charge you if you add ads to your site or produce more pageviews... yet... Let's create a "what if" scenario!

What if Akismet comes to you tomorrow or in a couple of weeks and says, "Hello, we're checking our accounts and we see that you're using the service since x months and it looks that your blog doesn't fall in the free category". You produce more 25k pageviews, you use ads... This makes x times $50 bucks. Will you pay ~1200 dollars, or whatever amount you settle? I don't want to take this risk and I think it's naive to hope that Automattic will never ever get the idea to check if all the free accounts are entitled to use the service for free.

About the privacy concerns: In Germany we a nice legal tool called Abmahnung (There's no English word for this, even the English wikipedia calls it http://en.wikipedia.org/wiki/Abmahnung) In short: A contender may send you a letter via his attorney to stop violating the law. If you comply and cease the violation, you have to pay his attorney costs, if you don't, you can end up at court. Many people pay... This "abmahn-business" is so successful, that some attorneys make a living out of it.