The page, which was available at http://wolframalpha.com/iphone, now redirects visitors to http://products.wolframalpha.com/iphone/, which is an information page related to their $50 iPhone app [iTunes Link].
Another page, http://www.wolframalpha.com/addtoyouriphone.html, shows iPhone users how to add the website to their iPhone (hint: just like any other page). Visiting the website in Mobile Safari brings up the image shown here, also suggesting that you buy the iPhone app.
The app gained wide attention when it was first announced due to its price tag, which critics said was far too high. Others rebuffed the criticism by saying that if you didn't want to use the app you could use the iPhone-friendly webpage.
Perhaps too many people did just that.
Thanks to TUAW reader Anil Doshi for the tip.
Update: Since there seems to be some confusion: yes, the regular website can be used once you dismiss the pop-up ad by tapping the "X" shown on the top-right corner of the ad. That seemed obvious to me, but obviously I should not make such assumptions.