Akismet for WordPress is an absolutely beautiful thing that keeps your posts clean of spam comments. However, it is a double sided sword that can keep quality comments out as well. I have experienced this first hand on a friends blog.
I have never left spam of any sorts in my life, however my IP was login and IP was flagged as such by the Akismet system. Apparently, I’m not the only one that has experienced this either, a quick search will yield others that have had the same problem. Therefore, all blog owners, manage your Akismet que, you might find some useful things in there. 
Akismet is one of my favorite plugins. I get a LOT of spam, and used to spend a lot of time deleting it.
Yep, Akismet normally finds upwards of 50-100 comments of spam per day on some of my blogs. This blog is more of a personal catch all of mine, but others, with a lot of backlinkage, tend to be the target of SPAMTASMS.