The reason that trumpeters and horn players have to do it in exams is that they meet the requirement frequently in orchestral playing. Cornet players occasionally have to play from parts for A cornet in the orchestra, but brass band parts are always for Bb.
QUOTE(cornetsrule @ Jul 27 2007, 08:37 PM)

yes, that was what was worrying me. transposing on the spot and transposing it while playing! a difficult task i would say
If you do enough orchestral playing, it becomes a lot easier. Classical parts are fairly straightforward, because the parts have rather few different notes, so you quickly learn the fingering that you use for them in a particular movement. It gets a bit trickier with early Wagner, in which the transposition keeps changing, and in Dvorak and R. Strauss, who wrote chromatic parts for various sizes of horn which get played on whatever you happen to own. This is usually an F horn, but even on a single Bb, you learn to play it from F parts, so that's how you think of it.
Some people with absolute pitch find transposition very difficult because the correspondence between what they see and what they hear is all wrong.