The Duolingo website says it's in private beta, which means it sifts through tons of e-mail addresses they get via the front-end page, probably on the look out for bogus addresses to screen them out, and then give out official invites to a few lucky ones.
Aside from being a language tutorial website, Duolingo doubles as a translation service. The developers are doing non-commercial content first, it seems. One of Duolingo's developers is Luis von Ahn of Carnegie Mellon University, who is known for bringing CAPTCHA and reCAPTCHA to the web. These 2 gating systems are common tools in many websites to deter bots and automated spammers. In fact, reCAPTCHA is perhaps very revolutionary for the Web that Google bought the system in 2009 for an undeclared amount; must be a pretty large sum for Google, a publicly traded company on NASDAQ, to hide details from the investing public.
Luis von Ahn at TEDxCMU
Who knows, I might be able to finally write a decent Spanish text after this module, or help out with the Spanish page of the TED Open Translation Project, of which I am currently a volunteer.
Evidently my knack for understanding español has dried up. It is quite impossible to cough up an extra hundred dollars to learn the language with other online services like Rosetta. Google Translate remains a free and viable option, but in my months of translating for TED, it's kinda hit-or-miss. The local Instituto Cervantes here in Manila is only a train ride away, but they charge several thousand pesos per module. I totally understand where they are coming from, requiring enrollment fees and all, having to earn their keep as a thriving language center, but of course I am hardly a rich man who can barely pay the rent.
I'm pretty eager to improve my syntax and expand my vocabulary like that of a Creole mestizo zapped out of the 1800's when the Philippines was still under Spanish rule. I think I'd be one lucky guy if I ever get Duolingo's official invite.