There are a lot of people who don't care for the new iTunes 10 icon. Several replacement icons have already been created. There was a previous hint about how to change the icon.
Another option is to use the freeware DockArt plug-in for iTunes is another option. It will replace the iTunes dock icon with the currently playing album cover. This plug-in has been around for a long time, but it still works perfectly with iTunes 10. The download includes documentation in a readme file, and the installation process given below in excerpted from that. There are several other options mentioned, so be sure to give it a read.
After downloading and unzipping the file, to install the plugin copy the file DockArtPlugin.bundle to ~/Library/iTunes/iTunes Plug-Ins (or /Library/iTunes/iTunes Plug-Ins if you'd like to make it available to all users).
If iTunes is running, stop and restart it. You'll see an entry named DockArt added to iTunes' Visualizer menu, but you don't have to select it. Any time you play a song that has cover art, it will replace the iTunes icon in your dock. When nothing is playing, or whatever is playing has no art, the normal iTunes icon will appear. The plugin will work, I believe, with all versions of iTunes from 4.1 onward but does require QuickTime 6.4 or later.
[crarko adds: I tested this, and it works as described. The DockArt plugin may have been around a while, but this is the first I'd heard of it. I like this better than the older hint because using a plugin does not require altering the contents of the iTune application package and running the risks associated with breaking the digital signature of the app. The plugin is mirrored (with permission) here.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100904095939774