The Article tells they use a ejabberd as backend and an extension of Jabber Web Chat as frontend to their web service. A Java client running with J2ME on all the new mobile phones completes the service.
When being online using the cellphone client it will override any web client for the user id logged in at the same time, by doing some priorisation stuff.
Over all this sounds pretty interesting. Anyway it's still unclear for me if mabber.com is yet another jabber server from which I can chat to all my jabber friends, or if this will be a closed user group with no interface to the remaining internet.
Pricing is also interesting: During beta the service is free, afterwards they'll they a montly usage fee for the service. So this gets really interesting only when UMTS/GPRS fees go down, too. And maybe someone should think about doing a regular open source jaber client which runs with J2ME...