Staffa - 10:37 pm on Nov 1, 2010 (gmt 0)
I saw the same request today.
The visitor from the US comes via a common search on Google.com and visits several pages like any other visitor to this site would do.
After loading the first page via the search result link there is this request for
/directory/url(data:image/png;base64,iVBORw [snip]) The [snip] is 1550 characters long !
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; GTB6.6; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729)
with referrer http : //www.mysite.com/initialdirectory/initialpage.asp and it gets a 404 (I broke the link)
The visitor keeps browsing the site in a normal pattern, accessing different directories and a number of pages, some of which are image heavy (in number but not in size), but no base64 requests.
Only when the visitor accesses the original landing page again is the request made (three times for this visit).
Anyway, it's quite apparent to me that it has nothing to do with any purposeful intention of the visitor and as long as it gets a 404 that's fine by me.
[Edit} Thank you grandma genie, the Wiki info explains it all ;o)