In a time of user centricity on the web, I have still not seen any web site or service that groks that time zone is not a per-author (or sometimes even per-visitor) configuration setting, but a function of present location, which in turn is a variable entity over time. (Hint: per-item, with a preference towards suggesting last used time zone, works a lot better. Do it in style, presenting it by the widget you have that shows creation time or similar, so it reads something like "June 25, 11:36, Europe/Stockholm time", relevant fields appropriately editable.)
I entered my flight details (I'm visiting San Francisco most of the later half of July) in Google Calendar the other day, but couldn't tie down start and end points in geography, so it shows my westward flights as ridiculously short, and my first-hop home flight in later July as a huge 24-hour stretch from San Francisco to Amsterdam. There are not always 24 hours in a day when in transit, but it's a very common assumption when crafting user interfaces. Of course it's a can of worms coming up with a good visualization of what time it is throughout the day, if you're in Sweden the first few hours of the day, in London by lunch and Vancouver at the end of the day, adding hours to the day every hop on the way.
Maybe FireEagle and cool hacks like this might pave the way towards betterization on this front, for the "here and now" kind of tools, like blogs, instant messengers and other social applications.
Good OpenID integration for the presence awareness broker would of course be another huge hit. That probably needs integration work from both sides, as the time for breaking some of the assumptions ranted about above comes upon us. I personally have a feeling this forthcoming user centric wave will hit the web from the client rather than server side, though.
These days, neither clients nor users are as dumb as they used to be treated as. But most server side code still is.