Wow, quite a bit of misinformation in this thread. OP, could you add an informational post about bitcoin to the beginning of the thread?
Bitcoins are a decentralized, psuedo-anonymous currency that was created a while ago by Satoshi Nakamodo. That's probably a psuedonym, but the guy was really smart, we do know that.
The bitcoin network solves the problem of trust in the banking system, in that no trust is required to ensure that more currency is printed than the average of 6 blocks an hour, and that coins are not double spent.
Essentially, coins are transferred to bitcoin addresses and the balance of that address is kept in the block chain. The block chain is what you are adding to when you generate a block, and each block includes transactions that you are verifying. A block is essentially a representation of a lot of work, which is what your GPUs and CPUs (god forbid) are doing, so in exchange, every time a block is created, the generator is allowed to add a transaction for 50 coins to an address they control - that is the incentive to help secure the network from double spending attacks.
What you are doing when you 'mine', is a collection of hashes and salts (I don't know the specifics offhand, but it can be thought of as running sha256 sums). Basically you run f(x), where f is the sha256 function and x is the nonce, essentially just an incremental counter, and if you find f(x) to be below a certain value, you have found a block! Congratulations! The value you are trying to get under is determined every 2016 blocks by every member of the network individually - the software tries to auto adjust to 1 block per 10 minutes.
There is no work to save, so you can shut down mining whenever you like. The Mhash/s value you get when you run a miner is telling you how many values of x you try per second, so for example 300mh/s is really running through that function 300 million times a second.
Bitcoins are not a fiat currency; at least according to wikipedia, a fiat currency is one issued by a government. Bitcoin is not issued by anyone, not a corporation, not a government, just people who wanted to solve the problems of trust in banking, and take back control. Bitcoin was designed to be deflationary (only slightly less than 21 million bitcoins will ever be produced) and uncounterfeitable, and in those respects it is very good at what it does.
Many stores do take bitcoins, with more joining all the time. If you like the idea of a secure, easy to spend and direct currency (rather than going through a payment processor or third party, as with paypal and credit cards) and there being no company to suddenly shut down and leave you stranded, or change the rules so that you suddenly lose everything or get scammed, then bitcoin is for you!
Finally, as for profitability, unless you get free electricity, you will likely lose money, especially as difficulty rises.
Read this post to understand why you will most likely not profit
Quickly, Nvidia is not as good because it has fewer stream processors, and how the architecture is setup is not as efficient as AMD (AMD's is harder to code for though, which is why Nvidia sometimes does better in games).
Oh and for those of you CPU mining... you will make very little for a lot of energy, and wear on your CPU. http://webcache.googleusercontent.co...www.google.com
And might I add, if you all had read the wiki or the forums, you'd have understood everything I just posted. I was linked by the bitcoin forums and was shocked to see how poorly people here understand bitcoins.
I need to stress, I suppose, that there is nobody controlling bitcoin! It is all community driven and completely transparent. You can go see any transaction you want at http://blockexplorer.com/