It disables the cache (preferable to give a short cache time), doesn't pass frames. That preg_match_all looks ugly, but wouldn't probably be run too much.

We are planning a collaboration campaign that will use random banners on the SiteNotice, and it will be used in some portals to generate random images and other contents.

Bryan.TongMinh wrote:

Any extension that generates random content, will unacceptably break the cache.

The best way to create a random selector without breaking the cache would be to write a JavaScript (gadget).