I'm interested in making work a LWM2M client on RIOT, I have a project in my mind.
For the moment the only solution I found to make it work on RIOT was to "translate" the client of leshan from java to c but that seems like a very complicated things, do you have any other ideas?
In a comment of 15. July 2014 you wrote that you didn't yet have any LWM2M implementation running in RIOT, but that was on your TODO list, is there some updates about that?
You should have a look at Wakaama, rather than Leshan. It is a C implementation of LwM2M. See https://github.com/eclipse/wakaama . If you have any question on Wakaama, the developer’s mailing list is at https://dev.eclipse.org/mailman/listinfo/wakaama-dev
AFAIK there will be a Google Summer of Code project to help with porting LwM2M to RIOT (hopefully using Wakaama) so you might want to follow that too (a quick search in the archives of the mailing lists will give you more info on the people involved).
Wakaama should run on RIOT, it's already running on Arduino & mbed.
Out of curiosity: since neither Arduino nor mbed are OS', providing a network
stack, what are you using for all the lower layers (CoAP, IPv6, 6lowpan...)?