It seems like the world should have passed this mark a long time ago but as of June 10th 2009, "Web 2.0", became the official 1 millionth word to enter the English language. It is defined as "[t]he next generation of web products and services, coming soon to a browser near you".
Other words that almost made the claim as the one millionth word were 999,999, "Jai Ho!", which was made popular from the movie Slum Dog millionaire. Also another notable word that is infamous in the gaming world, n00b, is officially word number 999,998. It's also the only mainstream English word that contains two numerals. N00b if often used to describe someone who is new to something, a newbie.
The Global Language Monitor who is responsible for listing new words that enter our language has strict criteria before a new word can be finally admitted to the language. According to the GLM a word must have a "minimum of 25,000 citations with the necessary breadth of geographic distribution and depth of citations" before it can be officially recognized.
As with any official counting there is always criticism that you can not truly count words and that the million word march is nothing more than fallacy. For example "the 600,000 species of mold are excluded, as are the tens millions of lesser known chemical substances, and the like" are not included as being countable words. Also, it was an estimate that got the GLM started and "GLM counts only headwords, so run, runs, and running are only counted once".
Even if you think the GLM is inaccurate it's the most accurate educated guess that we may have on how many words exist. Maybe a Web 2.0 pioneer will be able to create an algorithm using the web as its context to try and give us a better guess in the near future; but if he or she fails we can safely and accurately call him or her a n00b, finally.