I've been running a business focused on Bitcoin commerce and popularizing Bitcoin for 6 months.

I believe that the opportunities in mining are mostly past for current entrants in the space.

The opportunities to earn large capital gains through the increased valuation of speculative investments in bitcoin are certainly there, though very high risk.

Opportunities for earning cash flow, either in bitcoins or dollars, by providing services to bitcoin users are huge however, and provide the option of derisking by holding some of your income in dollars and some in bitcoin.

Current areas which have been hugely successful for some players are:
1) Bitcoin exchanges (bitstamp, mt. gox, btc-e)
2) Hosted online wallets/banks (coinbase, blockchain.info)
3) Bitcoin payment processing for e-commerce (bitpay, coinbase)
4) Gambling (Satoshi dice, satoshi circle, justdice)

Emerging areas:
1) eCommerce - Gyft is killing it, by acting almost as a point of sale exchange into gift cards. Opportunities are available for sellers willing to give discounts based on the expectation that bitcoin's value will rise.

2) Remittance - That is, moving money internationally. In some ways, Bitcoin handles this itself and will get better as exchanges with decent volume are developed for more local currencies. Though there is probably a great opportunity for a company that can use Bitcoin to make moving money from one country to another effortlessly. I really can't see how Western Union isn't totally screwed. Even if it adopted bitcoin, it would have to reduce it's cost structure tremendously to keep up with new entrants.

3) Safe storage - Bitcoin thefts from online wallets are still rampant. If you could create a wallet with the convenience of online, and guarantee, even INSURE peoples savings. You will win. Though that is a very hard thing to do.

4) More payment processing, specifically automated. Parking meters, laundromats, coke machines. It might be too early for that yet, but it could be pretty big.

That's enough for now. I've learned a lot from my time in the business, and am in contact with many of the major players.
Shoot me a line if you'd like to learn more.