alanback: Actually, the US has never switched to DST on the last Sunday of March. From 1987 to 2006, the US switched to DST on the first Sunday of April; from 1966 to 1986, the switch happened on the last Sunday of April (with the exception of 1974 and 1975 when the US switched in January and February).

So even under the old rules, there would still have been a week where just one side of the pond would be on DST.

Having said that, it's a pity that all you can choose from for a timezone is a fixed offset of GMT (although if it would really be a fixed offset of GMT, everyone would have to switch their timezone twice a year as GMT is a fixed timezone without DST). There are publicly available databases with rules for all timezones (the Olson database for instance). But that would lead to a huge drop down list, and many people would get confused and start complaining.