DVB-T transmissions have already started in some countries (UK, Australia, Germany, Finland, Italy, France, Spain) (DK started 31 March-06, DR1, DR2, TV2). France is due to begin transmitting Pay Per View broadcasts in September 2005 (in MPEG-4).
Depending on the transmission mode, about 15 Mb/s can get transmitted per transponder (frequency). This is enough for one HDTV channel or four to five standard-definition TV channels. Multiple channels can be organized in bouquets (a bunch of services by a particular service provider).
Crazy as it may sound, if you find that video and audio sometimes freeze (and there are no error messages in /var/log/messages), your aerial signal may be too weak. A signal amplifier made a significant difference in my setup (Hauppauge Nova-T (Conexant) + Avermedia 771 DVB-T, 2.6.13-rc7 + dvb kernel patches from CVS)