Skype Ltd. has been trying to sell itself as a serious business tool since early 2006. It’s an ambitious strategy, and it represents a big jump from the company’s roots as a pioneer of free Internet calling for individuals.

It has also proven to be a much harder sell than expected.

At the beginning of its campaign, Skype targeted small and midsized businesses (SMBs). Its first step was to provide corporate billing and administration capabilities, so that companies wouldn’t have to reimburse employees individually for their use of Skype for cut-rate outbound calling. The next big step was interoperability with Asterisk and other open-source IP PBX platforms, another emerging SMB money-saver.

In early 2009 it added Skype for SIP, which made its service accessible through any certified SIP-capable IP PBX. And recently it began offering multiparty video conferencing capability for up to five users.

Skype’s most recent move came in mid-May, when it introduced an upgraded Web-based administration interface for enterprise IT managers, dubbed Skype Manager. After a free trial period that lasts through October of this year, Skype will introduce a per-user charge for using the interface.

But while all of these developments make Skype increasingly easy for companies large and small to use, the question remains: Will enterprises ever fully accept Skype as a mainstream business tool? My prediction: They probably won’t for some time, but they should.

There are a number of wholly understandable reasons an enterprise might want to avoid Skype. One is its image as a service mainly used by cash-strapped college students, immigrants, and travelers — or by small companies that can’t afford “real” commercial phone services.

There are also practical concerns about IT infrastructure. Like all VoIP services, Skype adds IP traffic to the corporate broadband connection, and represents yet one more app to be administered and tracked.

The worst part, though, may be the uncertainty Skype introduces into an organization’s IT management and security processes. Skype sneaks through firewalls without permission using unknown methods. Its encryption methods are also a mystery. In fact, no one knows much of anything about how Skype works. And because it’s always adding new features, there’s no way to predict how employees will use it. In short, it’s hard to know how Skype use will affect any given company.

But there are also good reasons to use Skype. For one, it lets employees receive Skype calls from people around the world who might hesitate to call if they had to pay international long-distance rates. It can also save money on outbound calling, though that advantage is shrinking.

Skype also lets employees make video calls to one another without major equipment expenditures -– all it takes is a Webcam. And it provides them with a convenient, widely used way to do file transfers and IM chat (although both could introduce additional security concerns).

In addition, while Skype’s business offerings have traditionally targeted SMBs, the new Manager has no restrictions on the number of users and a simple online form can boost your corporate limit on credit purchases.

Ironically, the strongest arguments in favor of Skype are also the main arguments against it. In practical terms, for example, Skype is probably just as secure as other enterprise communication methods. A couple of years ago I did a bit of digging into Skype security, focusing on how easy it would be for governments to tap Skype calls. With the help of a couple of famous cryptology pioneers, I tentatively concluded that even with Skype’s permission and cooperation, such tapping would be difficult, and that much harder for private individuals or groups. (Disclaimer: This does not constitute professional or legal advice).

Either way, there have been almost no reported breaches of Skype security.

Similarly, Skype’s unpredictability means that employees can use it to find truly new ways to communicate. They might hold multi-party video calls that solve problems. They might use free desktop- or file-sharing Web meeting services that have integrated Skype voice capability. And given Skype’s continuing development, opportunities for such ad hoc communication methods should only increase.

While this means some loss of control for IT, it could also produce big rewards in terms of employee creativity and corporate flexibility. That alone makes Skype worth considering.

Unfortunately, corporate realities usually dictate erring on the side of caution. As a result, I predict Skype will remain an enterprise outsider for at least several more years.

Or will it? What do you think?