Currently we just silently fail to cache things if $wgShowIPinHeader is on, which it is by default.
This is pretty lame and error-prone, since it's realllly easy not to realize you have to turn off this cosmetic setting to make caching work. I take full responsibility for making it work this way back in the early 2000s, but that was dumb of me. :)
We should just force it to off if eg $wgUseFileCache is in use, since that's the far more interesting/important setting.
Version: unspecified
Severity: enhancement
URL: http://lists.wikimedia.org/pipermail/wikitech-l/2009-January/041125.html