Well, one point still remains. The difficulty is going to double during 2012. So at some point here it will halve. This calculation includes cheap $0.07/kWh electricity, so it would take a long time to pay off a card.
The doubling is interesting (assuming you are taslking about when the bounty is reduced to 20 coins), because it's not the same as a difficulty increase. A difficulty increase is intended to keep the overall rate of coin generation the same, just accounting for new hardware on the scene. The halving of the bounty, however, will reduce the total output of coins by 50%, for everyone.
Thinking about this in a supply/demand sort of way, I see a few scenarios-
1- Mass miner exodus, as all the miners who were barely breaking even are now losing money to mine. Difficulty drops over time, eventually equilibrium is reached where if you are in an area with cheaper electricity you can make some money.
2- Coin supply cut in half, miners are selling half as many coins as before, coin value shoots up. Value might not double exactly, but coin value will go up enough to keep many miners in business, else it reverts into scenario 1 above.
3- Some combination of the two above.
4- Worst case scenario, all the miners who quit dump all their current coins on the market, saying "F this", market temporarily flooded so value of BTC goes down instead of up as I would expect. This causes more miners to quit, as mining profit is even worse, chain reaction, until only a small fraction of the original miners remain. With such a small mining population, it takes a long long time to reach the next difficulty change. When it's reached, mining becomes nearly profitable again, but it's too late, the damage has been done.
I don't think the 4th scenario is very realistic, but it is a small possibility and so I wouldn't bet the house on bitcoins.
In other news, I've installed a bitcoin miner on my llano laptop just out of curiosity as to what the APU can do. 32 mhash, about what I'd expect. I need to see how much power it draws from the wall while mining before I make a call on if it's worth mining with it or not.