I would love the ability to choose from a few different date/time formats (some choices spelling out the month would be great) rather than just sticking to one convention. I recognise 8 Sept, 2009 much more easily than 09/08/2009, personally, and I'm sure there are others that would prefer different formats. I think I still make friends' heads asplode when I write 25/12/2009 for this year's Christmas date
Displaying time in 24hr notation would be a fantastic option as well, although that's less of an issue for me.