I listen to music through hard-to-insert-and-remove
isolating ear phones, and would like to be able to
take cell-phone calls without removing them.

My cell phone only fits standard 2.5mm headsets that
include their own mono speakers and microphone.

I'd love references to commercially available adapters
costing less (or much less) than $100 and/or a 
description of how to create my own or solve my problem 
in another way.

A good solution would include 
- an outlet for standard stereo earphones
- an outlet for a standard microphone or a built-in microphone

An ideal solution would be highly portable but this is not a requirement.

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=42-2496
and 
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-373

About $10 plus tax. In store, you might find Gold Plated for higher
fidelity. :) But, this will work enough for you, I think.

Well, Sorta. I forgot about the microphone bit. What are you going to
do about the Microphone? It'll be another dangly bit all told. Since
Microphones and speakers are (can be?) similar in construction, you
*might* be able to use the headphones where one ear is listen and the
other is microphone. Still, you'll have to contend with shutting off
music while talking on the phone. :).

Heck, for $10, it's not so bad. 

You could go bonkers and also pick up
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-948
which can be used with
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=17-952
... but we've gone too far, now, haven't we? (If you get this, the
other parts probably aren't necessary.)

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=17-953
is an option to just clip the mic portion near your mouth...

Well, I tried splitting the 3/32" jack into two 3/32" jacks
with 274-948 (accepts two 3/32" stereo phone plugs, fits 3/32"
stereo phone jack).

Into the first jack I placed a standard 3/32" cell phone 
headphone (which I only used for the mic).

In the second, I placed 274-373 ( accepts 1/8" stereo phone plug, 
fits stereo 3/32" phone jack) and connected my earphones through
there.

After a bit of wiggling I could hear audio through one ear and
input audio through my microphone while my earphones were on.

The solution was fragile, and the connections weren't very good.

I ended up buying 17-855 (wireless phone recording controller, 
accepts 3/32" cellphone headphone plug, fits standard cellphone
3/32" jacks, has a mono microphone out plug, requires 2 AA batteries).

I have the microphone out go into my computer, and my headphones
coming out of my computer, and turn the unit on and off to take
a call.  The sound comes through both ears and the quality is very
good.

This is exactly the solution that I needed, and cost about $25.00.

  http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F008%5F001%5F012%5F000&product%5Fid=17%2D855&site=search

Thanks a bunch for your help - it got me on the right track - and
did technically meet my requirements.  If you post an answer, I'll
be happy to pay.

I am not a Google Answers Researcher, but I am glad I could help.
Actually, you did almost exactly what I thought you might...

I'm very glad you found a solution for your question. GA Researchers
have hyperlinked names, and only they can post answers for payment.
Since you have your question answered, you might wish to cancel the
question.

I guess I'm all set then. Thank you so much!