Steve Ballmer doesn't use one but apparently as many as 10,000 Microsoft employees do. Embarrassingly, the device in question is Apple's iPhone.

The Wall Street Journal claims to have uncovered evidence that the Apple iPhone is now so successful, large numbers of Microsoft staff are using it instead of a device running Microsoft's recently re-launched Windows Mobile even though they do not necessarily have a 'development' reason to do so.

The estimate of 10,000 employee users came from unnamed sources who heard it quoted by executives as the volume of people accessing the Microsoft email system while using the device. Long suspected to be popular, this is the first time a credible figure has been put on iPhone use inside the company.

The popularity of the iPhone is so galling to Microsoft chief Ballmer that he is said to have taken one from an employee who was using it to snap pictures at a recent Microsoft event, and mimed stomping on it in front of other attendees to show his displeasure.

Microsoft has since allegedly changed its mobile phone reimbursement policy to exclude devices not running the company's software, the paper says.

After recently launching the Windows Mobile 6.5 refresh of what is generally agreed to be stodgy fare on offer from the company at the moment, the company is pinning its hopes on a new Smartphone platform, Windows Phone 7, due out later this year or early in 2011. The change poses risks - the new OS will be incompatible with today's software and there is also a question mark over the precise timescale for its release.