Hm, lots of confusion regarding transposition here ...
Saxes and trumpets can be Eb or Bb, meaning that if you ask them to give you a C, you will hear an Eb or Bb as it is on a piano. If you play a lot in a jazz setting, you'll get used to playing in F, since it is D or G for the horns (with just 1 or 2 sharps - but on a 4 string you can't really hit that all important Eb underneath, don't you hate it). Try getting them to play some guitar-oriented changes, and their brain will have to work overtime.
Having PP, I have no problem detuning on bass or guitar. However, my brain will not allow me to refer to the open Eb as an open E (in my mind) so I have to do a little transposition in speaking. I can sing from notes from non-C tuning, but I have to transpose the notes I'm reading (no guarantees for any pleasant sounding notes in my singing, but that's another bucket of something). And if I try playing a keyboard that is tuned too low, my fingers can get lost completely.