Yes, the problem is that an attacker can do fradulent purchases of bitcoins, this can be done in several ways, then when it's time for the operator to pay out to the bitcoin service, it's the bitcoins service that will be out of pocket.
1. An attacker get's credits attached to a cell phone number illegally.
2. He buys bitcoins.
3. Time goes by....
4. Fraud is discovered.
5. Operator does not pay out to payment processor.
6. Payment processor does not pay to merchant.
7. Merchant has lost bitcoins and has received no funds.
I've developed a solution that interfaced with Impulsepay in the UK, it gives you coins instantly when texting a code.
You can read more about it here: https://bitcointalk.org/index.php?topic=64793.20
However, we have discontinued the feature.
The idea was to make bitcoins easily accessible, but while texting to get a ringtone is ok, the incentive to cheat (get free money) is simply too high when dealing with bitcoins. To cover the losses from fradulent purchases and to pay fees charged by operators and payment processors, the surcharge gets ridiculously high. And to prevent fradulent purchases, we could put various restrictions in place, but it would defeat the whole concept of 'easy of use'. Since we don't know for 45 days after a purchase was made whether it was legit or not, it's very hard to know how much of a loss we're sitting on.
There could be methods for verifying a customer, but the more hassle it is to become verified, the harder it becomes to get those coins in the first place. After all, if you just want a bitcoin, would you really scan your passport and an utility bill just to get one by texting ?
In the end I guess it's a matter of holding the pros and cons against each other, and determine which level of risk is acceptable, and how much customers can be inconvenienced without hurting sales too much.
Another point is that there are several other methods that are a lot cheaper when purchasing bitcoins, so making SMS purchases just as elaborate as a few other methods while the whole point was 'easy of use', this coupled with the fraud risk, makes sales of bitcoins for mobile payments essentially dead.
Currently, it's possible to use https://blockchain.info/wallet/sms-phone-deposits
(I'm not associated with that one).
They're limiting number of purchases to 1 pr. customer pr. month. In my view this goes against 'ease of use', as it's a great way to get somebody introduced to bitcoin, but if you need more than one purchase of coins, you're out of luck.
And what if a malicious individual breaks into the website of a cell operator and get access to customer accounts, usually he would be able to send SMS'es from there, thus purchasing a lot of coins from numbers he doesn't own. To avoid detection, this could even be done slowly over a period of maybe 30 days. 5 fradulent purchases a day for a month would add up to 150 BTC or 777 USD with current prices. So, there should be limitations at the bitcoin service, that would reduce the possible loss.
The only entity that I could see could run something like this successfully would be someone making a lot of money on other services, and that would be willing to take the risk with the SMS/mobile payment service.
Once malicious individuals finds a loophole to get free bitcoins, it's going to be exploited, you can be pretty sure about that.
I think the only services/digital goods that are suitable for SMS purchases are those that can be easily copied without a direct monetary loss to the operator. Example: I sell a certain ring tone, if somebody scams me and I don't get paid for 10% of all sales, it doesn't matter much as I have no direct loss of giving out 'free' copies of the ringtone. The ringtone itself is just some bits and can be copied indefinitly, however, a bitcoin cannot be copied, and once it's transferred, it's not possible to get back easily. The only way would be to contact the received and ask gently to get it back, good luck with that when a blackhat in Rio just robbed you for 500 USD. :\
So, as much as I'd like to offer bitcoin purchases over mobile payment, I've come to the conclusion it's not a viable business model, as bitcoin transactions are irreversible, while mobile payments are not.. It's for the exact same reason most people won't accept PayPal or CC for bitcoins.
And don't misinterpret me, I would love for it to be a viable business model to do mobile payments to get bitcoins, and I'm interested in all feedback in regards to this, personally I'm not willing to take the risk at the moment though.