Custom Feed with Multiple Categories

How do you go about creating a custom feed that outputs multiple categories in WordPress 2.7?

In the past, http://www.uxbooth.com/feed/?cat=11,12 has worked (This example would link to a feed showing posts from category 11 and 12.)

However, now this only redirects to the first category listed (in the above example, 11). This has worked in previous versions of WP including 2.6.

Is there a new way to do this?