Exactly backwards!rufus said:yes, it depends on the cassette, and, though i'm not an expert, probably the greater use of compact chainrings will minimize that somewhat. .
if you use a cassette with bigger cogs in the back, such as the Campy 13-29, or a Shimano 12-27, you'll need a longer cage to allow for the greater chainwrap required and to keep the chain tensioned properly. all the manufacturers will specify what cage length is required for what cassette you use.
now, i'm just guessing here, as i haven't really looked into it much, but i would suspect, that if you were to use a compact chainset, such as a 34/50 combo instead of a 39/53, that you might be able to run a short cage derailleur with the 12-27 combo, as the fewer teeth on the front chainrings will allow you to meet the chainwrap figures specified. but i'd need confirmation from someone like kerry Irons on that matter.
i know Campy recomends at least a mid cage derailleur with their 13-29 cassette, but i also know that people claim you can use a short cage perfectly well, as long as you avoid the severe cross-chained positions(53-29 combo)