I don't have anything recent on this issue, but here is the gist
of the discussion taken from:
Lieberman E. Risk factors for uterine rupture during a trial of
labor after cesarean. Clin Obstet Gynecol 2001;44(3):609-21.
This review is discussing a study (Rozenberg
P, Goffinet F, PHillippe HG, et al. Ultrasonographic measurement of
lower uterine segment to assess risk of defects of scarred uterus.
Lancet 1996;347:281-4.) in which thickness of the uterine scar
in late pregnancy was correlated with likelihood of scar rupture.
The reviewer notes that using a cut-off of 2.5 mm or less would be
a reasonable threshold in that only 8% of the study population had
a scar thickness this thin and 10% of them experienced a scar
rupture. However, the reviewer also notes that the overall scar
rupture rate in the study population was 2.3%. That tells you that
the results of this study cannot be generalized to a population
that received proper care because any number of VBAC studies have
reported scar rupture rates of 0.5% or less.
If you follow the link for Rozenberg to the PubMed site, you
will be able to use the "related articles" function to see other,
more recent research on this issue.