According to Sony's Satoshi Hashimoto firmware 3.0 will not remove the ability for owners of the older, fatter, and shinier PS3's to install alternative operating systems.
Andriasang provides a translation of Japanese site Impress Watch's interview in which Hashimoto in which he says as much. He also apparently "assured readers that there currently isn't a problem with the feature leaving a security hole with the system."
Among other updates which you can read about in the link provided, the report also states: "I'm certain some people have figured this out already, but the slimline PS3 does not ship with System 3.0. This was due to circumstances during manufacturing. Slim buyers will have to update like the rest of us."