2008.1 / Gnome / ALSA, fully updated.
System sounds are definitely enabled, I've selected various audio files for system events (e.g. login / logout) from the defaults, but ( a ) they won't play when previewed and ( b ) they don't play during the relevant events.
All other audio works fine (e.g. Nautilus mouseover previews, mplayer handles whatever I throw at it, and a well-known video sharing website), so I'm puzzled why it's so reluctant to play system sounds.