I think he was just passing on a message from vanitygen
OK, my bad, corrected statement is probably: Prefix '3GiG' not possible
using the current version of vanitygenNo, it's genuinely not valid. The first digit of any given Bitcoin address is the "version byte."
It "wastes" 2 bits (64 and 1) to accomplish aesthetic norms. Bit 64 *could* be
assigned in the future if we ever have a "crunch". By using the high bit (128)
to designate test networks, all testnet addresses will now begin with '2'.
Bitcoin script-hash (aka OP_EVAL) addresses are assigned version 5 (using the
aesthetic +1), which means they always begin with '3'. Signatures are on
version 10 and/or 11, beginning with '5'.
A standard Bitcoin address will always start with a 1, testnet addresses start with a 2. Addresses starting with a 3 are OP_EVAL addresses (for a description of OP_EVAL addresses, check
this Bitcoin StackExchange question/answer). The prefix tells Bitcoin how to handle transactions involving that address, so using an OP_EVAL prefix for a standard address could cause problems in interpretation - though I've never actually seen what happens if you actually try.
There are several
other prefixes in use, often indicating that an address belongs to an alternate blockchain like Namecoin or Litecoin, as well as one prefix indicating that you're looking at a Bitcoin privkey.
While not every Bitcoin client performs
proper sanity checks, they
should require a destination address beginning with 1 or 3 and they
should require a special form of signature to re-spend coins sent to an OP_EVAL address that I don't think the default client currently supports.