Kernel 2.6.28 already has network namespaces indeed, however they do not support sysfs. Thus, it is not usable as is in Fremantle. In theory, you could back-port sysfs namespaces patches from 2.6.29. This would imply flashing a new kernel image, and possibly breaking the kernel module ABI, thus flashing a new rootfs as well. In other words, it would be a usability nightmare.

In my humble but informed opinion, you're much better off doing TCP/IP in userspace for the MMS GPRS context. The kernel pieces are already in place for this: the pn_pep kernel module can provide either a virtual network device, but also a plain sequenced packets socket. In the latter case, you can read and write raw IP packets in isolation.