License

You need to have look into, at least:
ejabberd
Erlang
MySQL (if you have that) etc.

Only thing I know (which doesn't mean that there isn't any more with special licenses) is MySQL. It is The GNU General Public License(GPL), which is not as "nice" for "secret" companies like The GNU Lesser General Public License(LGPL) (GNU clib is one with LGPL license). That means that IF you use MySQL libraries and don't buy license from them, you MUST deliver YOUR source to every one that YOU deliver binaries to. Same goes for your customers too. Can then go to others with that to make other changes etc...
If they had used LGPL on there libraries, you could have had any license you wnat on your code.
Read http://www.mysql.com/ for info about MySQL.
Read http://www.gnu.org/licenses/licenses.html for info on different GPL licenses.

According to http://www.erlang.org/, Ericsson calls Erlangs license Erlang Public License (EPL). It suppose to be like Mozilla license.
See http://www.erlang.org/EPLICENSE and http://www.erlang.org/license/EPL1x0-explained.html.

Looking in ejabberd's tarball (tar archive) looking into the file COPYING, you can se that it is GNU General Public License version 2 (GPL). So same as for MySQL, without the posibilities like MySQL.

So, if you drop MySQL, I guess that you could use ejabberd as you like, as long you don't add code to core ejabberd (GPL license). Would guess that modules would need to be licensed GNU to,
If you add to core, you have to make the source available to your customers (but no need to deliver to others, but the customers can).

For a a fee, I could tell you more. This took me 6 min to find the information and write down...