robzilla - 7:27 am on Jul 25, 2008 (gmt 0)

I've had a closer look at the log files and noticed that these requests almost always occur shortly after a page is visited by MSNBot. That same page, as well the linked CSS file, are then requested by a seemingly automated process that uses a "human-like" user-agent, as I mentioned in my earlier post, and an IP such as:
65.55.109.190, 65.55.110.194, 65.55.110.149, 65.55.110.158, 65.55.110.73, 65.55.110.86, 65.55.110.210, 65.55.110.183, etc.

After that, no other requests are made; no images loaded, no other pages visited, which strengthens my belief that these are not human requests, but automated ones, apparently looking merely at the HTML and CSS of your pages. If it is indeed part of a quality check, which seems likely to me, cloaked pages might be something they're looking for, along with, perhaps, hidden text, and other spammy techniques.