The tech press was abuzz this week with rumours from seemingly well-placed sources that social network giant Facebook was developing a mobile phone designed to compete head-to-head against Apple’s iPhone, Google’s Android OS and Microsoft’s Windows Phone 7 offerings.

Facebook attempted to quash the story early on with a curiously-specific statement of; “we are not building a phone”, which only served to add fuel to the rumour as it followed a similar tone to Google’s claims during the development of the now-established Android mobile OS. The inference being of course that Facebook could still be preparing some sort of an assault on the mobile market with third-party device manufacturers such as HTC and Samsung doing the actual phone ‘building’.

With Facebook’s dominant and constantly-growing position on the social networking scene and 500m+ registered members already sold on the web, a mobile OS based around the service would certainly make sense to a vast number of people.

While it remains to be seen whether there’s any truth in the Facebook-phone rumours, the story adds yet more weight to the viewpoint that mobile is the key area in which the major players will battle for the foreseeable future.

Although smartphones have been around in some form for a number of years, it’s only now that they’re finding their way into the pockets of the average mobile user, and they’re certainly not limited to the tech-savvy corner of the market as previously. The functionality of the latest batch of competing smartphones – full internet, html email, apps, GPS, HD video recording and playback, hi-res digital cameras, music, video calling, organisers and of course voice calls – underlines just how far the technology has come, along with how many other areas it is successfully invading.

The landscape of the mobile market these days means that the decision for the consumer isn’t as simple as choosing a phone anymore, it’s more often than not a 24 month commitment to a company and its operating system, and everything that comes with it… and it’s not hard to see why Facebook would want their share of that.

The humble subject line has long been an overlooked element of email marketing, often taking a back seat to design, copywriting, imagery and call-to-action refinement during the development of a campaign.

Yet, despite its lowly status on this list of specialist areas, the subject line has actually always been an important factor in email marketing. A good subject line, basically, can make the difference between an email flyer being opened and acted upon or ignored altogether. You can have the best looking email marketing in the world but without a decent subject line to back it up it’s never likely to fulfil its potential.

Now, with mobile devices becoming ever-popular, the subject line is more important than ever to email marketing because of one simple fact;

Smartphone inboxes only display subject lines and from fields.

No images. No preview panes. It’s all down to your subject line. So it better be good.

With the explosion of smartphone use in recent years – the ubiquitous iPhone and Blackberry, along with the rapidly phenomenal Android OS phones (which grew 350% in the last quarter here in the UK) – more and more of us are accessing our emails on the move, or even opting to use smartphones as in-office business devices due to their ever-increasing performance benefits. Smartphones aren’t the preserve of business or the technically-minded, they’re for everyone. And it seems everyone has got one.

Microsoft, which has stayed on the fringes of the smartphone market for the last few years, is moving closer to the launch of its “Windows Phone 7″ operating system – a hi-spec platform that will doubtlessly be backed up by a gargantuan marketing budget when it emerges before 2010 is out. The new Windows offering will present users with yet another super-capable handheld option – and it’s only going to get bigger.

Of course, email on the move simply presents yet another way to use email marketing to communicate effectively and efficiently with your target market. So it’s unquestionably a good thing.

But next time you’re crafting a campaign that will spring into life on iPhones, Androids and as yet unimagined super devices, make sure you don’t forget the importance of the element that has been around from day one of email – the subject line.