Here is a part of this mail in the mailing list from Satoshi:
The bandwidth might not be as prohibitive as you think. A typical transaction would be about 400 bytes (ECC is nicely compact). Each transaction has to be broadcast twice, so lets say 1KB per transaction. Visa processed 37 billion transactions in FY2008, or an average of 100 million transactions per day. That many transactions would take 100GB of bandwidth, or the size of 12 DVD or 2 HD quality movies, or about $18 worth of bandwidth at current prices.
I don't quite understand why every transaction is broadcast twice. The first time clearly is when the client broadcast its transaction to the network, but what's the second time? (Is it considering broadcast of the mined block itself the second time because it contains the transaction?)