i wouldn't recommend an adapter as it will put additional strain on the phone aside from adding additional length to the connection, can make it very bulky in the pocket- i had to do the same several yrs ago and would never use an adapter for a phone again (not that i'd ever get a phone with a 2.5mm plug again)
I would agree with the Bluetooth comment as the sound transfer is generally very good. Unfortunately many are quite bulky and you can not get the in ear type. I tried a 2.5mm-3.5mm stereo adapter cable ($4 on trademe) on my Nokia E71 It works ok but seems to create a bit of noise that you do not get on the 2.5mm buds supplied with the phone (which unfortunately are rubbish otherwise)
My Bose headset came with a 3.5mm->2.5mm adaptor which I use with my Nokia E51 and there is no annoyance or noticeable degradation in sound quality. I don't see why such an adaptor purchased as a cheap accessory would perform any differently.
Thanks Canterbury. I imagine being Sennheiser they would cost a few dollars. About the extra noise with some adaptors.. I think it has something do do with the fact the 2.5mm phone sockets (as any other phone socket) are designed for stereo plus a mic and some of the stereo only adaptors do not line up properly with the internal connections of the phone creating a bit of interference.
What kind of adapters are you talking about? I think DSE only has the chunky ones that sit on the end of the cable, useless in my opinion. The ones on trade me I've seen are smaller with a short cable attached which seems like it'd be better. Alasta, what is the Bose one like?