Microsoft is set to radically change the face of mobile phone applications. The company is planning to allow Windows Mobile 6.5 customers to run its applications on more than one mobile phone, allowing friend, family and colleague to share applications. Microsoft has said that customers of its upcoming Windows Marketplace for Mobile would be able to run purchased apps on as many as five Windows Mobile phones at the same time.

Marketplace customers will also be able to get a no-questions-asked refund on an application provided it is within 24 hours of purchase, said Daniel Bouie, a senior product planner for Microsoft.

Microsoft is gunning for rival smartphones with these customer-friendly moves, especially Apple's iPhone, whose owners today can only download the same app onto multiple iPhones if they all have the same credit card and iTunes account informations. That would appear to rule out easy sharing of apps.

The marketplace is expected to debut this autumn when Windows Mobile 6.5 smartphones start to become available.

Besides Apple's store, the marketplace will contend with online app markets such as Nokia's Ovi Store, RIM's BlackBerry App World, Google's Android Market and Palm's App Catalog for the upcoming Pre.

While application-sharing will appeal to users, it could turn off Windows Mobile developers, said independent analyst Jack Gold.

"Developers want to sell as many apps as possible. They don't want you to run a single app you bought on multiple devices," Gold said.

Apple's upcoming iPhone operating system 3.0 will reportedly allow owners to more easily switch between App Store accounts. That could make it easier to share apps between users, though they may be required to share their passwords, too.

That feature appears to be less smooth than Windows Marketplace, which will require customers to register the phones for which apps will be shared, Bouie said.

Apps can be downloaded as many times as needed, to re-install on replacement or upgraded phones, provided users don't go over the five-phone limit, Bouie said.

Microsoft will use its LiveID authentication system for tracking. "It will be very hard for the casual, semi-casual or semi-pro user to pirate apps," Bouie said.

He disagreed that app sharing will anger the developers Microsoft needs to woo.

"We feel comfortable that using our LiveID system to help connect products to five devices is a great balance of the needs of both developers and end users," Bouie said. "We see this as a permanent feature, and we've gotten great positive feedback from the vast majority of developers we've talked with about this."

There are more than 20,000 apps available for Windows Mobile phones today on independent sites such as Handango.com. But more than 25,000 apps are available for the iPhone after less than two years.

To get on the good side of developers, Microsoft is trying to position the marketplace as a more open and transparent alternative to Apple's App Store.

Microsoft earlier this month published guidelines governing what apps it will allow.

Also, developers accepted for Microsoft's marketplace will be allowed to sell the same wares via other stores, Bouie said.

The App Store is the only authorised channel for iPhone apps, though there is a thriving ecosystem of those providing "jailbroken" apps.

Microsoft is working on letting developers sell subscriptions to apps, though it won't be ready by the Marketplace's launch, which will take place in 29 countries.

"I can't commit to a date, but it will be one of the first things we do" after launch, Bouie said.