the first day of 25C3 has been great fun. I attended 3 talks: PLC (the power line stuff), 202c, iPhone dev-team, and SS7. I must say the SS7 talk was the best. The iPhone talk was boring (maybe they showed something interesting in the last 10 minutes but I left before the end).
The congress is really packed with people, they sold all tickets on the first day (3800).