Some of the new features in Windows Vista are well hidden but useful in specific circumstances. A little thing like the clock on the taskbar has a nice improvement that you might not notice: You can add a second (or third) clock to the display to keep track of the current time in a different part of the world. This comes in handy if you’re a telecommuter who has to keep in touch with the home office on the other side of the country, or if you build web applications and need to keep track of timestamps in GMT.
Start by clicking the clock in the taskbar. That opens up this display, showing your system time and the time zone you entered during setup.
To add a second clock, click the Date and Time Settings link and click the Additional Clocks tab. Select the Additional Clock 1 check box and then choose a time zone. Give the new clock a label, if you wish.
After you save your changes, click the taskbar clock again to see the new clock in action.
Because this change doesn’t affect the system date or time, any user can customize the setting this way. You don’t need to be an administrator to make the change – and thus no User Account Control dialog boxes will appear.
Ed Bott 
I haven’t receieve my Vista Beta 2 yet, but here’s a question: Is there a way to put the date next to the time (i.e., 6/7 11:43 AM) without making the taskbar two lines high? I’ve been using 1st Clock to do this for years, but I wish it was built in to Windows.
Nope, you can show or hide the clock, just as before, but you can’t customize its appearance.