From "MLA Task Force on Evaluating Scholarship," Profession 2007, pp. 27-28:
Estimated percentage of English and Foreign Languages Doctorate Recipients Who Become Tenure-Track Faculty Members and Achieve Tenure at the Institution Where Hired
Completed doctorate degree: 100
Hired to tenure-track position within five years: 60
Considered for institution at institution where hired: 38*
Awarded tenure at institution where hired: 34
*According to the report: "The MLA survey indicates that the largest number leave one tenure-track appointment for another. We have no way of tracking tenure outcomes for these junior faculty members" (27).
For a reasoned analysis, read the report. The short version of the "Is the glass half full or half empty?" question seems to be that if you get a position, as 40% will not, and are considered for tenure at the same institution, you're pretty likely to get it. I'm not sure if this counteracts any of the gloom about MLA interviews and job anxiety, but I hope so. It's still a bad market.
Updated to add: Miriam Burstein has a good summary of several of the articles.