When the Nokia Communicator was launched in 1996 it was the first commercial product to combine the functionality of a mobile phone with that of a personal digital assistant (PDA), including electronic calendar and address book capabilities. Its introduction led pundits to predict that the mobile phone and electronic organiser would soon merge into a single device.

But, while later versions of the Communicator have seen continuing improvements in the technology - the next one will include a full-colour screen and redesigned keyboard - the market for a converged phone/PDA has remained niche.

"By the end of last year around a million communicators had been sold," says Carsten Schmidt, associate analyst at Forrester Research. "When compared to the overall market for, on the one hand, Palm devices, and on the other, GSM phones, the figure is tiny."

Nokia alone manufactured 128m mobile phones last year. And with entry-level prices now about £10 or the equivalent in many European markets, many have found the £250 price-tag of the Communicator an expensive way of combining a phone with an electronic organiser.

But the fundamental problem, says Mr Schmidt, is that people baulk at the cost of a stand-alone PDA, let alone one bundled with a phone. "The reality is that the PDA remains an absolutely overpriced address book and calendar."

Until recently, therefore, Nokia's competitors have been reluctant to follow its lead. However, recent excitement over the mobile internet, and the opportunity to add e-mail, Wireless application protocol (Wap) and data communications to mobile devices, has put convergence firmly back on the agenda.

Mobile phone manufacturers, for instance, are introducing smartphone devices, such as the Kyocera smartphone and the Ericsson R380 - which combine phone and PDA-like functionality, plus Wap, e-mail and messaging services.

And PDA manufacturers have begun to add voice and data communications to the traditional electronic organiser. Last December, for instance, saw the US launch of the Handspring VisorPhone, a clip-on mobile phone that attaches to the PDA's expansion slot, and costs $299.

But the clearest evidence that the market for converged devices is heating up is Microsoft's decision to develop a new smartphone platform codenamed Stinger. Stinger will support voice and data communications, including GPRS - the so-called 2.5G wireless packet switched service - as well as pocket versions of Microsoft's popular Outlook and Internet Explorer applications.

So, is the mobile phone finally converging with the PDA? "Absolutely," says Jeremy Gittins, group marketing manager, strategy business, at Microsoft UK. "And broadband wireless is the killer technology that will really kick it off. It will see the whole wireless industry being built around data traffic to personal devices."

But, while faster networks will help, successful convergence will depend on a number of other factors, too, not least the ability of manufacturers to integrate the functionality of phone and PDA into a single device - while at the same time retaining the weight, useability and compactness that users expect. Pricing will also continue to be an issue.

Herein lies a challenge, concedes Richard Ward, head of UK operations at mobile phone manufacturer Trium. "The characteristics of both devices do not all come in a small package, so there is always going to be a trade-off between things like screen size, weight, and battery consumption," he says.

And, as the complexity and functionality of the devices increases, so does the challenge. PDA users, for instance, increasingly expect a colour screen. "But", says, Mr Ward, "colour PDAs are inevitably much bigger, heavier and more expensive than mono screen ones."

One way of mitigating this problem is to us