It was only a short time after the first edition of the MI text (Miller & Rollnick, 1991) was published that researchers got going, running controlled trials in all sorts of settings.
This is on the one hand impressive, on the other problematic. The number of published trials now exceeds 160, apparently, and the rate of publication is rising considerably each year. I shant summarise this evidence here, because you can examine the bibliography in www.motivationalinterview.org or look at one of these reviews, which I think point to a conclusion: there's something in MI that works, across a range of behaviours and problem areas, but we are not quite sure what and why.
Dunn C, DeRoo L, Rivara F (2001) The use of brief interventions adapted from motivational interviewing across behavioral domains: A systematic review. Addiction, 96: 1725-42
Burke B, Arkowitz H, and Dunn C (2002) The efficacy of motivational interviewing and its adaptations: What we know so far. In Miller W, and Rollnick S (eds.) Motivational Interviewing: Preparing people for change (second edition). New York, Guilford Press.
Burke B, Arkowitz H and Menchola M (2003) The efficacy of motivational interviewing: A meta-analysis of controlled clinical trials. Journal of Consulting and Clinical Psychology, 71, 843-61
Britt E, Hudson S, Blampied N (2004) Motivational interviewing in health settings: A review. Patient Education and Counseling, 52, 147-55
Rubak S, Sandboek A, Lauritzen T and Christensen B (2005) Motivational interviewing: A systematic review and meta-analysis. British Journal of General Practice, 55, 305-12
Hettema J, Steele J, Miller WR (2005) Motivational Interviewing. Annual Review of Clinical Psychology, 1, 91-111.
Further Information
The trouble with such rapid expansion has not surprisingly been limitations on a number of fronts. Perhaps the most serious is the very small number of studies that paid attention to exactly what method was put to the test. Most were adaptations of one kind or another, but few provided evidence about the skilfulness or otherwise of the practitioners who delivered the intervention.
One very useful parallel development has been the construction of reliable and valid measures of competence (MITI; MISC; BECCI, for example) and the detailed study of process. The names to look out for on the latter subject are Paul Amrhein and Terri Moyers, both colleagues of Bill Miller's in Albuquerque.
I'll add to this little piece of you post a more specific query about research in the Discussion Forum.