Update: Swannman suggested a simple-yet-effective method I'd completely missed for proof: user agent strings! Without further adieu, the proof is in the pudding:
-- Safari 3 on 10.5.1 reports:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-us)
AppleWebKit/523.10.3 (KHTML, like Gecko)
-- MobileSafari on iPhone 1.1.2 reports:
Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en-us)
AppleWebKit/420.1 (KHTML, like Gecko)
So while it looks like MobileSafari on 1.1.2 may not be running the latest-and-greatest version of WebKit, it is definitely endowed with a version in the 3.0 family, and one I'd gather has the majority of the speed increases touted in the "10 new things" post.
Now the really interesting question: what user agent does MobileSafari prior to 1.1.2 purport to be? Anyone with an older iPhone OS version can quickly hit this link and fire an email my way with the results, so we can put this one to rest once and for all.