Please pardon my lateness in responding.
I seem to remember that when this thread began, the original questioner was
asking about TEMPORARY walls, and might have meant modular wall units.
I heartily agree that drywall over plywood is a great system for permanent
walls.  Personally, I would scrimp and save to come up with the money for at
least 3/4 in. plywood, as screws driven into it will hold heavy
artifacts/display cases much more securely than 1/2 in. plywood.
Temporary walls are another question entirely.  They need to be easily
storable, because few if any museums have adequate space for storing unused
exhibit furniture between shows, they need to be cost-effective, and they
need to be easy to work with.  It's a tall order that I haven't seen filled
yet.
Thre are a lot of systems that seem really great for trade expos, but I
haven't seen one yet that looked ideal for temporary museum exhibitions.
 Everywhere I've worked has had a system that I thought was very flawed.
 Portable walls tend to be either too heavy to be portable, or too portable
to be securely installed, or sometimes both.  Building and demolishing
cheaply made "permanent" walls for every show is cost-inefficient, makes a
hell of a mess in the galleries, and often looks cheap.
My latest thinking leans towards a system that is similar to the commercial
trade show wall units, with frames that you re-use indefinitely and facing
material that is interchangeable for different styles and can be cheaply
thrown away when worn out.
Since I know of nothing that fits the bill, I am going to start designing and
building my own in the near future.  My recommendation is that you look at
the temporary wall systems available (many of the companies will send a
salesman to your facility and demonstrate them for you) think about their
flaws, and see if you can come up with something specifically tailored to
your institution, and have it custom-built by a local fabricator.  Given the
cost of many commercially-made systems, custom work may seem cheap.
Good luck and feel free to contact me for brainstorming.
-Adam
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The ideas expressed herein probably bear no resemblance whatsoever to the
official wishes, policies, and public image desired by the Atwater Kent
Museum.   Don't be getting any wacky ideas that I speak for the place just
because I work there.