In its biannual “Taking Stock With Teens” survey, Piper Jaffray reported that the iPhone remains popular among U.S. teenagers — although it’s not as hot as it was immediately following the launch of the iPhone 3G last July. Sixteen percent of those surveyed in March planned to buy an iPhone in the next six months, down from 22% in October.

The number of teens who actually own iPhones, however, remained flat at about 8%. Piper Jaffray’s Andrew Murphy attributes the descrepancy between those who aspire to and those who actually own iPhones to parental sticker shock at AT&T’s (T) rates. “Teens definitely want the iPhone,” says Murphy, “but expensive data plans may be the limiting factor in parents’ minds.”

The survey of adults, conducted in mid-March by ChangeWave Research, showed a similar pattern, but at a significantly higher level of interest. Among the 4,292 relatively well-heeled cell phone owners who responded, 37% planned to buy a BlackBerry in the next six months, 30% planned to buy an iPhone and 4% planned to buy a Palm.

As research director Paul Carton notes, interest in Palm — having almost flat-lined — is growing measurably in advance of the expected release of the Palm Pre. Interest in the iPhone peaked last summer, when 56% of ChangeWave types said they planned to buy the new model. Interest in RIM’s BlackBerries peaked, although not as sharply, in December, after the release of its new models.

Asked about the new iPhone 3.0 operating system that Apple has said it will release this summer, one in five ChangeWave respondents said they’d be even more likely to buy an iPhone in the future. When asked specifically about possible new iPhones and price points, they returned these results:

• 9% said they are likely to buy a 32GB iPhone 3G ($299)
• 11% said they are likely to buy a 16GB iPhone 3G ($199)
• 8% said they are likely to buy the 8GB Traditional iPhone ($99)

All of which bodes well for Apple, according to Carton.

Palm has a tougher challenge, he says. Not only are BlackBerry and iPhone owners remarkably loyal to their providers (only 4% of RIM customers and 1% of Apple customers said they were likely to switch to a Palm), but Palm has tied its fate to Sprint, a carrier that only 1% of respondents want to switch to.

Back in the teen survey, Murphy attributes much of the iPhone’s popularity to the penetration of the iPod and iTunes Store among this cohort. The iPod’s market share has held steady at 86% over the past 12 months, and although only 19% of teens planned to buy a new MP3 player this coming year (down from 34% six months ago), 100% of those who did planned to buy iPods.

Meanwhile, iTunes now enjoys a 97% market share among teens — up from 81% a year ago — with No. 2 RealNetworks hanging on at 2%.

“Apple’s dominance in the consumer electronics and online music markets is going seemingly unchecked,” wrote Piper Jaffray’s Gene Munster in a separate report to clients. “We believe that the teen demographic is a critical  component of long-term growth in the digital music and mobile markets, and Apple is taking its leading position in music and moving  aggressively into the mobile market.”

The recession didn’t seem to play a large role in either the Piper Jaffray or ChangeWave reports, although neither survey focused on the poor or recently unemployed. The main group of 600 teens surveyed by Piper Jaffray enjoyed an average household income of $73,000; another 9% had household incomes of $100,000.

The ChangeWave survey, according to its literature, is drawn from “a group of 20,000 highly qualified business, technology, and medical professionals — as well as early adopter consumers — who work in leading companies of select industries. They are credentialed professionals who spend their everyday lives on the frontline of technological change.”

They are not, presumably, standing in the unemployment lines.

[Follow Philip Elmer-DeWitt on Twitter @ philiped]