I bought a two "HOSA" 3.5mm to 2.5mm headset adapters -- they cost almost $9 each with shipping.
They don't work with Emily's BlackBerry -- we only hear sound on one channel.
I then checked the BB site -- they sell their own adapter. Alas, an Amazon reviewer found the same problem with BlackBerry's own adapter!.
Extremely annoying. I wonder what the iPhone does.
2/4/08: I had slightly different results testing on a Nokia 6555b. I also noted that the Bose Communications Kit comes with four different 3.5mm to 2.5mm adapters -- each for different phones. So a 2.5mm to 3.5mm adapter may have to phone specific.