If you do a "cost analysis" in terms of purchase price vs revenue generated, then:

ASSUMPTIONS
- do not factor in electricity costs
- BTC exchange at today's rate of $120 USD/BTC
- network difficulty on average increases 10% every 2 weeks, thereby reducing revenue by about 10% every 2 weeks

GPU
- AMD 7970 at stock speeds provides 650 MH/s
- Cost $400 USD or 3.33 BTC
- Generates 0.0209 BTC/day at today's difficulty
- After one year of continuous mining total BTC generated = 2.74 BTC
- LOSS OF 0.60 BTC

USB Block Erupter
- 333 MH/s
- Cost 1.99 BTC
- Generates 0.0107 BTC/day at today's difficulty
- After one year of continuous mining total BTC generated = 1.40 BTC
- LOSS OF 0.59 BTC

BFL Jalapeno
- 5000 MH/s
- Cost $274 USD = 2.28 BTC
- Generates 0.1611 BTC/day at today's difficulty
- After one year of continuous mining total BTC generated = 21.1 BTC if you start mining today!
- If you receive your unit and start mining 6 months from now
   - Generates 0.0454 BTC/day
   - After one year from that point, total BTC generated = 5.945 BTC

AM-PT SHARE
- Currently about 2.5 BTC/share at BTC-TC
- Recently generated 0.038 BTC/week in dividends (this was the highest dividend ever)
- After one year of dividends at this rate = 1.98 BTC
- LOSS OF 0.52 BTC

In other words, if you purchase any of the above items in the hopes of "earning back" your cost, you won't (with exception of BFL units). Of course this does not include the fact that you can sell your 7970 GPU in the future (but a year from now how much would it be worth? maybe $100?). Others would say you cannot sell the USB Block Erupter after a year because it is a uni-tasker without any other purpose. The BFL units are reasonably priced. Problem is they are not shipping in bulk, so a delay in shipment leads to large loss in profit. Finally, you can sell your AM-PT share too, and it is very difficult to predict what the sell price would be at that time.