Google and Yahoo have several competing services. To see if their organic results are biased, i searched for the following on yahoo, google, and msn:

[video]
Google:
#1: Google Video
#8: Yahoo video search

Yahoo:
#1: Yahoo music videos
#11: Google video (second page)

MSN:
#1: Google video
#33: Yahoo Homepage

[news]
Google:
#1: CNN
#4: Google News
#7: Yahoo News
#8: MSNBC

Yahoo:
#1: CNN
#3: Yahoo
#6: MSNBC
#7: Google News

MSN:
#1: Google News
#2: MSNBC
#3: CNN
#6: Yahoo News

[images]
Google:
#1: Google Image Search
#70: Yahoo Image Search

Yahoo:
#1: Yahoo image search
#2: Google Image Search

MSN:
#1: imagecomics.com
#3: Google Image search
(Yahoo not in first 25 pgs)

Im too tired to do any more

Hah! Good find. From these, it looks like only MSN is truly objective. Then again, you could also say "Google has a certain philosophy to ranking which they also apply to their products, which is why they rank better". Same argument could work for Yahoo :)
It doesn't work for all things tho so it could be a coincidence. Enter "shopping" or "videos" or "maps" into Google and you will see Yahoo's listed before Google... I can't find any important word right now for which Yahoo lets Google to be first, though :)