Hi Fabio,
For what i read now Osmocom have 2 software BTS (Osmo-BTS and Soft-BTS), communicating with the A-BIS over IP interface to OpenBSC: http://lists.gnumonks.org/pipermail/openbsc/2011-March/002529.html http://lists.gnumonks.org/pipermail/openbsc/2010-April/001508.html
Now we have two software BTS in developing phase, softBTS and osmoBTS, In A-bis side, both communicating with OpenBSC using ipaccess style A-bis over IP protocol. Probably difference would be in Um side. I am not considering Universal Software Radio Peripheral as hardware for softBTS, that mean no plans to use openBTS as bridge to USRP.
fadil