The latest update to Safari is a few days old. It came on the heels of a problem playing YouTube videos, when all I could see was an Obsolete Flash comment and no video or way to bypass the message. This affected only Safari, and getting an update to Flash using the link in YouTube didn't fix.
The next day the 4.0.5 update came out so I installed that. Didn't help, but it didn't hurt anything else so it's okay to get that update if you are currently running any version of Safari 4, or 3.2.
Leopard users have to be running 10.5.8 to use Safari 4. I still prefer 10.5.6 as being trouble-free.
To fix the YouTube problem I went on YouTube's forum areas and asked what was going on. Others reported it too, and the 3rd answer was the fix:
Open Safari preferences
Click on the Security button
Click on the Show Cookies button under the Accept Cookies area.
The next window has every cookie saved. Click on the Search field, upper right corner.
Type in YouTube. This should reduce the list to less than a page.
Click on Remove All.
Click on Done.
Make sure the 3rd button "Only from sites I visit" is the one selected.
Close Preferences. YouTube videos should play properly now.
Note to Snow Leopard users: 10.6.3 release is imminent. Apple will probably release it at the same time as the new laptops some time this month. As always, do NOT let Software Update install this. Wait at least a week and I and others will have had time to test it for problems.
When it's time to install, go to http://www.apple.com/downloads and search for 10.6.3 Combo Updater. Download the Disk Image (.dmg) file and run the updater from there. As always, run Disk Utility first and Repair Permissions, and Verify Disk. After the installation Repair Permissions again.