The basic problem is that Apple’s implementation of SMS shows the sender’s ‘reply to’ number and not the originating number. But senders are able to change the reply to details to whatever they wish. “Most carriers don't check this part of the message,” says pod2g, famous for his role in jailbreaking iOS, “which means one can write whatever he wants in this section : a special number like 911, or the number of somebody else.”
This would allow scammers, phishers and criminals of various flavors to socially engineer the user. With an altered reply to header, the recipient could believe it’s a message from a trusted friend, when really it’s a criminal recommending this wonderful new (but covertly malicious) website. On the internet, one wrong click can be disastrous.
Engadget contacted Apple for its response, and was told, “One of the limitations of SMS is that it allows messages to be sent with spoofed addresses to any phone, so we urge customers to be extremely careful if they're directed to an unknown website or address over SMS.” It said that "When using iMessage instead of SMS, addresses are verified which protects against these kinds of spoofing attacks."
In short, if you use an iPhone for your SMS, it will be more secure to use iMessage. But whatever device you use, be careful with SMS; and if you cannot see the sender as opposed to just the reply to number, be extra careful.