Let's find out. Since you didn't mention which BFL device you purchased or how much you pay for electricity, I'll just use their avg cost per GH as a base ($50/1GH @ 4.6 watts) and $0.15/KWh. Assuming the difficulty is 150 million when (if) you receive your unit in October and 1 btc still = $75, you will start off making 0.02347 btc/week/GH ($1.64 * # of GH in you BFL miner - per week after electricity costs).
Now we have to guess what the rate of difficulty increase will be from that point onward. The last few increases have been as high as 25% per ~11 days. I don't know how long that rate will be sustainable - but I'll guess that difficulty will still be increasing at about 50% per month (slightly slower than now). That means that your BFL miner will cease to bring in more money than it costs for electricity after about 28 weeks. Total btc mined after 28 weeks would be ~0.27btc/GH (~$20/GH @ $75/btc).
In short, your $50/GH investment would make a total of $20/GH which is a 60% loss. Of course, all of these numbers are hypothetical. If you want to play around with a profitability calculator, I would recommend
this one.
