Quoting "Hans Hagen Outside" :
-- was pragma@wxs.nl on Aug 29, 2005 at 06:50:51PM --
With the next pdftex we will not need this active mess any more since pre-char spacing will be supported natively; by that time we need to adapt the ffr files This is a good news indeed. By the way, does this include the possibility to treat the character in different ways depending on the current mode (math/text)? Because, in the current setting, if you request for 3cm before `:' and then define a mathematical function $f : R \to Z$ you'll end up with a huuuge space between f and : which you certainly isn't what you want.
Thanks for your answer, Olivier PS: Also the default definition in useencoding is .25em for :;?! I'm no typographical expert at all but I thought the standard is indeed .25em for ;?! but a normal space before `:'