I think Apple means that if an advertiser targets and serves up their own ads (using a newly modified API), Apple will no longer want a cut of whatever the advertiser pays per view or click. Because Apple won't be involved at all. Only the app developer.
So I don't think it matters much to developers. If anything, their cut might go down since advertisers normally pay less than what Apple wanted.
However, if an advertiser goes through Apple's iAds servers, in order to target ads using Apple's information on us, then Apple will still keep 30% of the fee paid by advertisers... which pays for the servers and for providing the targeted customers.
I could easily be wrong, but after poking around, that's the impression I get. Apple's not crazy. They're not going to give away a service for free. They just no longer want to be involved in vetting ads, or actively hunting for advertisers.
(I've always thought that iAds was simply another prong of Jobs' "thermonuclear" war. That he intended to get into search, ads, maps, whatever Google did.)