Orkut has a Doodle up, Google has only changed the alt-tag of the logo to wish you a happy valentine's day. (Also mentioned by George in the Olympic thread.) Yahoo and Ask have Valentine's day themes, Bing not.

http://1.bp.blogspot.com/_Ln0bvERl9jA/S3MFSJMrC0I/AAAAAAAAAGg/b5ccC0Ec9dM/s320/orkut_doodle_valentines_day.gif

http://s.ytimg.com/yt/img/logo-valentines2010-vfl145896.png

https://img1.orkut.com/img/doodle/orkut_doodle_ValentinesDay_b_v3.gif

To make it complete: Combined logo with a heart and Valentine alt-text.

http://www.google.com/logos/olympics10-prsskating-hp.png

You guys notice everything... ;-D

Ask.com start page
(with mobile browser, sorry):

http://m.ask.com/wap4?app=ask&c=x&sid=10-10-10-235-ask-1266188818904&crt=MAINTEXT&ch=1266188960288&id=country=US-entry=askweb-&trc=&NEXT=MAIN&aid=GRAPHICAL

Bing.com has skating theme:

http://www.bing.com/hp?&MKT=en-us

Ask.com's Desktop start page:

http://www.ask.com/

This is what we found a year ago:

http://blogoscoped.com/forum/150486.html

Finally from Ask.com:

http://sp.ask.com/sh/i/a11/hp/sk/hpto_vday10_01.jpg

From Bing.com (theme has changed):

http://www.bing.com/fd/hpk2/WomensMoguls_EN-US1584584395.jpg

And Yahoo! start page's Valentine's Day theme is based to Flash (can't paste here).

The last one:

http://www.baidu.com/

(Baidu.com)

http://s.ytimg.com/yt/img/logo-valentines2010-vfl145896.png :)

YouTube was already mentioned here.