According to the Windows 8 Preview FAQ, media centre is not included with Preview by default. It can easily be returned by the method shown below.
Some websites are claiming that once the final version is released you'll have to pay for the use of media centre. As to how true this is I'm unsure but will do some checking and report back.


Windows Media Center is not preinstalled in Windows 8 Release Preview. If you want to use Windows Media Center, you need to add it by following these steps:


1.
Swipe in from the right edge of the screen, and then tap Search.
(If you're using a mouse, point to the upper-right corner of the screen, and then click Search.)

2.
Enter add features in the search box, and then tap or click Add features to Windows 8.

3.
Tap or click I already have a product key.

4.
Enter this product key: MBFBV-W3DP2-2MVKN-PJCQD-KKTF7 and then click Next.

5.
Select the checkbox to accept the license terms and then click Add features.

Your PC will restart and Windows Media Center will now be on your PC and the tile will be pinned to the Start screen.
Reference:
Windows 8 Release Preview: frequently asked questions