Are the new services being developed by Apple (NSDQ: AAPL), Nokia (NYSE: NOK), Yahoo! (NSDQ: YHOO), Sony (NYSE: SNE) Ericsson (NSDQ: ERIC), Google (NSDQ: GOOG) and others threatening the carrier’s walled garden?

Yes, according to a JupiterResearch report. But it doesn’t have to be a threat if carriers embrace new alternatives, the report concluded. The question is an important one as big-name companies with well-known brands start getting into the mobile content business. Already, handset manufacturers, such as Nokia and Sony Ericsson, are pursuing content strategies; Apple is bypassing the carrier by having users upload music and other content to the phone through iTunes; and Internet and media companies are experimenting with services that go around the carrier because they believe fees are too high to drive market uptake.

Jupiter examined European consumers who side-load content to their phones from the PC compared to overall mobile users and online users. Thomas Husson, who led the research, reported that operators should not prevent side-loading, but partner with these new players. If the carriers attempt to block side-loading, they may alienate these valuable users. It found that mobile users who side-load are more likely to pay for mobile content (10 percent vs. 3 percent of overall mobile users) and for digital music (19 percent vs. 6 percent of overall online users).

Among its recommendations: carriers should launch services that aren’t restricted to the mobile phone, but also work on the PC, which can boost revenues, and they should partner with third-party players, whose customers may be more knowledgeable in downloading content.