Sorry, I meant naked date/times that are displayed on a page. It might be that site A gets the user's local time zone correct, but site B, C, and D don't. By displaying the time zone abbreviation next to the displayed time it removes all ambiguity and possibility for error. Also, if it's a time zone abbreviation that's displayed (e.g. EDT) make sure to mark this up properly and fully explain which time zone and country this is for. I can't remember which, but there are some time zones that have the same initialisms. Some don't have initialisms at all, and users from, say, Poland might not know what PDT is either.
This is what we did on a recent project. The times were left at UTC with a hover tooltip (JS with a fallback to title="Coordinated Universal Time") and we used the "timeago" plugin for jQuery http://timeago.yarp.com .
