They’re generally additive. Not all sites will need both, but the speedup from each one is independent — there aren’t any antagonistic effects between the two.
Adding support for XCache on PHP 5.3 is on our roadmap, but we’ve got some other stuff in the pipeline first (most notably, the PHP/FastCGI changes I alluded to earlier). XCache loses a lot of its benefits when run under our current FastCGI configuration, as the cache ends up not being shared between processes. It worked better when PHP was configured as an Apache module (mod_php), but we won’t be continuing to support that beyond PHP 5.2.
(Why? Because there’s a number of limitations to mod_php — it runs with the wrong user’s permissions, only one version can be loaded at a time, it doesn’t load user-specific or domain-specific PHP.ini files, and it generally just works differently from everything else. Plus, it can’t be used at all on a shared server.)
To the best of my knowledge, APC isn’t being integrated into PHP at any point in the near future — it’s still a separate extension in PHP 5.4.0, which is likely to be released in the next few months. We do recognize that APC is becoming the “standard” caching extension nowadays, though, and we may start using it instead of XCache in the future.
With regard to updating Page Speed — I believe we’ve fallen a bit behind at the moment, but updates should be on the way.