The stream data is stored within a transaction output using a special OP_DROP + OP_RETURN format. We will document this on the site in due course but here's the basic format:
spke [16 byte prefix of createtxid] OP_DROP spkk [binary item key up to 256 chars] OP_DROP OP_RETURN [possibly long binary item value]
The stream item is written in a transaction so it enjoys the same protection (digital signature + immutability) as any other blockchain transaction.