It is a world wide number extrapolated from a 10,000 person survey. The headline numbers are a tease for their $2500.00 report. Forrester seems like a reputable company but survey questions are tough to properly craft.
From an Information Week article by Michael Endler | February 08, 2013 01:28 PM .
Surface Pro Demand: Don't Believe The Hype
"The figure is an extrapolation of data collected for Forrester's 2013 Mobile Workforce Adoption Trends, which surveyed almost 10,000 information workers in 16 countries and found that 32% of respondents want Windows running on their next work tablet."
and later ....
"In an interview, Forrester analyst Ted Schadler said it's possible demand for a Windows 8 tablet has declined and acknowledged that Surface Pro might not be the device survey respondents envisioned. He countered, however, that the "main point is that people thought and likely still think that Microsoft is a contender in tablets." This point is valid; Redmond certainly has the reputation and resources to inspire anticipation and confidence among would-be tablet buyers. The question, though, is whether Surface Pro will be the device that actually translates these sentiments into sales. "