I have found the feature that lets me activate this again on the new board software -- this allows people to come online as 'invisible' so their name doesn't show up in the listings of 'Who's Online'. For various reasons some people like to go 'invisible'. If you want to use this setting, you can tick it in your User CP.

NB: While you WILL appear as invisible to registered members, you are not invisible to me or the moderators as we can see who is online (and we can see that you are set to invisible). Obviously this shouldn't matter to most people using the setting, but just in case people are wondering...