I've noticed since the release of the IE8 Beta 2 some strange request for non-existing pages: the requests are something like this "http://www.mydomain.com/(null)"
The only thing in common between those request is the user agent, always IE8 using the Trident 4 engine:
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0)
It might include other toolbars or add-ons, but that's the common part for all of them. Most are using XP, but there is also some Vista requests.
The only page that I've been able to find so far is this thread that is focused just in how to ignore those request, instead of trying to understand the reason why are generated.
I'm afraid that now that the final IE8 has been finally released the number of such bogus "(null)" requests will increase at a very fast pace, so the only solution will be to set a rule to ignore it as just another bug in IE8 and don't get flooded with the 404 error.