I measured a 4mm when my sonographer did it. Dr. Rozenberg's 1996 study on this measurement suggested a cut-off of 3.5mm; because I had a higher measurement than this, I felt comfortable going forward with my VBAC attempt.
Here's the link to the description of the 1996 study.
Description of how Dr. Rozenberg performs the measurement:
From: [Dr. Rozenberg]
Sent: Wednesday, August 1, 2007 12:28:00 PM
Subject: RE how to perform ultrasonographic measurement of the lower uterine segment
Between 36 and 38 weeks
Full bladder (visualization of the whole LUS)
Sagittal sections of the LUS
Measure of the thinnest zone (upper third of the LUS)
Calipers at the interface of the bladder and the amniotic fluid-decidua
LUS in an almost horizontal plane
3 measurements (smallest value chosen)
PR(See attached file: SI.5.3.jpg)