Here is a depressing calculation for you all
Lets say that difficulty keeps creeping up at 10% pace. After 6 months, we are looking at diff somewhere around 6613326 and new shiny USB powered coffee warmer arrives. 200 USD and 3.5 GH/s + 40 USD for S&H or what ever.
If diff keeps going up at the same rate per month, this is what happens:
(BTC @ 6 USD)
Month BTC USD Difficulty
1 14.74 88.44 8002124.46
2 11.69 70.14 10650827.6563
3 9.15 54.9 12887501.4641
4 7.26 43.56 17153264.4487
5 5.68 34.08 20755449.9829
6 4.51 27.06 27625503.9272
7 3.53 21.18 33426859.752
8 2.8 16.8 44491150.3299
9 2.19 13.14 53834291.8991
10 1.74 10.44 71653442.5178
11 1.36 8.16 86700665.4465
12 1.08 6.48 115398585.709
Total 65.74 394.44 USD
If diff moves up faster, when you start mining with ASIC, lets say 20% per month on average, numbers start to look even more depressing:
Month BTC USD Difficulty
1 13.53 81.18 9523189.44
2 8.24 49.44 16456071.3523
3 5.04 30.24 28436091.2968
4 3.07 18.42 40947971.4674
5 1.84 11.04 70758094.6957
6 1.13 6.78 122269987.634
7 0.69 4.14 211282538.632
8 0.42 2.52 304246855.63
9 0.25 1.5 525738566.528
10 0.15 0.9 908476242.961
11 0.09 0.54 1569846947.84
12 0.06 0.36 2260579604.88
Total 34.51 207.06
Do not forget to subtract your cost of power.
I used
http://striketeam.ath.cx/btccalc/btccalc.php for the numbers