This is due to repeated cases where someone pays for Bitcoins with Paypal, receives their Bitcoins, and then fraudulently complains to Paypal that they never received their goods. Paypal too often sides with the fraudulent buyer in this case, and so exchangers no longer allow this method of funding.
quote: He said that because the 'attack' you're talking about doesn't make any sense. This theoretical attacker wouldn't have access to anyone's bitcoin wallet, just their bitcoin addresses, which are public keys, which are not vulnerable to an attack of the kind you're proposing.
quote: Based on the current difficulty of 567269.530162, the network hash rate can be approximated at 4 THashes/sec (tera-hashes). Given an average CPU mining rate of 5 MHashes/sec (this is a high average too), it would take over 800,000 CPUs to match this.