Why doesn't Samsung Kies show an update for my Galaxy Ace? My phone has Android 2.3.3, build GINGERBREAD.XWPKY. I have the latest version of Kies and my phone isn't rooted. But I believe I should see an upgrade to 2.3.5 there. Any clues?

Note: When I first plugged my phone into Kies, it found a firmware update. I wasn't very sure what it was, but after the update the version is still 2.3.3 (which is the version before I did that update.)
My phone is also on 2.3.3. The updates get pushed out depending on a number of factors including region (as mentioned by Crashhh in his comment) and carrier.

From my understanding, often the releases will be controlled by the manufacturer (and in the case of Samsung and most others, customisation occurs). Once the manufacturer is ready to support it in a particular region, it will release the software to the carriers. Many carriers will also make their own modifications (and add their own bloatware). Only once this process is complete, will the update be available in Kies.

With the release of Gingerbread in Australia from my carrier, I know that the release went back and forth between Samsung and the carrier a few times, delaying its release to customers.

I'm speculating, but the update you received may have been some sort of patch to the manufacturer/carrier specific software, and therefore I wouldn't expect it to increment the version number of the OS.