As more people subscribe to cell phone services–nearly 220 million in the U.S as of June, according to the CTIA Wireless Association–marketers see the mobile market as a ripe opportunity. According to research firm Informa, marketers will spend more than $11 billion on mobile advertising by 2011. Some of the marketing is being done through legitimate channels. Companies such as eBay and Orbitz allow customers to sign up for services that send text message alerts. Cell phone operators are also starting to experiment with sending text messages promoting new services. But if mobile operators want to exploit this marketing opportunity, they must tread lightly so as not to annoy customers with messages they don’t want, experts say. And a recent rise in text message spam could jeopardize these efforts.