Apparently, SlightlyShadySEO is very shady. He noticed that his site was manually reviewed by Google and took the time to blog about it. He writes:
Manual reviews are Google’s awknowledgment that anything algorithmic will have flaws. ... The manual review I stumbled across today was one that appears to be centered around cloaking and [possibly] link spam.
By examining specific referrer URLs, he assumes that Google is doing certain searches based on webmaster spam reports or assumptions of cloaking. He's also concluded that since the visits were rather slow (and came from Google IPs), they had to have been done by humans employed by Google.
The entire post is an insightful read, and if SlightlyShadySEO is correct, is very enlightening about the process of manual reviews.
Forum discussion continues at Sphinn.