I had a quick look on Google for v6test.yahoo.com, and what I see suggests this is to do with the move from IPv4 to IPv6.
IP is your Internet Protocol address and has been issued by your ISP under IPv4. But those IP addresses are running out, and a new version IPv6 has been introduced which allows many more IP addresses to be allocated.
Many ISPs are not yet ready for the change over, and what you see may be Yahoo's test to see if you are using IPv6 yet.
The link itself, v6test.yahoo.com/eng/test/eye-test.png is just to an image file, (test.png), so I suspect they are testing whether they can download an image to your browser via any IPv6 connection. As they cannot, (because you are likely still using IPv4), the image fails to load.
There's more about IPv6 in my post here;
So in my view, nothing to worry about, it is just Yahoo testing.
I notice starting on Friday May 27, 2011 that when I go to yahoo.com everything will download for the page except this one item.
(1 item remaining) downloading picture http:// v6test.yahoo.com/eng/test/eye-test.png
I'll leave this page up, but even after 10 minutes this still has not loaded What is this and what is happening? is it an infection? My security scans show no infection