@AC AC re. virtual currencies
Paypal is a payment management system that does not issue 'currency' and e-gold is an investment system that allows you to buy/sell gold investments but does not issue a currency. They are supposed to operate under various government guidelines for customer background checking to avoid being used as a money laundering channel.
Games like Second Life operate an in-game token system where tokens can be bought and sold by the game operator for real world currency. Tokens are exchanged between players as part of game activity and also between players and the game operator for game services. I know that SL players will say that it's not a game but as far as the money flow is concerned, it operates as a game of skill where you can win money from other players where the money is in the form of tokens issued by the game operator.
These tokens can be freely transferred between players but can never be taken outside the game and so do not count as currency. It is possible to use SL for money laundering but SL places limits on transaction levels and operates a distinction between 'ordinary' accounts and 'merchant' accounts, with transaction limits to detect and prevent this.
A debit card is a tool, under the control of the banks, that enables money in your bank account to be transferred to someone else's bank account under your instructions, so it's nowhere near being a currency. A gift certificate or shop voucher, which is issued by a retailer organisation, is in fact a form of currency (technically) but it is time limited for redemption and there are not many people who would be happy to accept a shop voucher as payment for goods/services; so as such it is useless as a currency.
The problem with 'alternative currencies' as far as governments are concerned, is the possibility of money laundering and forgery. The 'official' banking system has many procedures and checks in place to detect and prevent this but any alternative currency or virtual currency would probably not have this high level of oversight due to the cost of setting up and maintaining it.