This one stumped me…
At a particular client site, they have mixed versions of Office (2003 and 2007) on their desktops, and have just recently embraced Sharepoint’s datasheet view for bulk editing lists of client, suppliers etc.
Well, today one of them mentioned that certain columns weren't appearing in their datasheet view, but they were on other computers.
As it turns out, it appears that Lookup Fields and People Picker columns don't work in the datasheet view of the Office 2003 install.
A real bummer, so my only workaround was to use the choice fields (drop down list)